Add reactive flag to Place

This is part of a stack for inferring variables which are reactive via *control 
dependencies* as opposed to a data dependency. In compiler engineering, a 
statement S2 is control-dependent on statement S1 if S1 is in the post-dominance 
frontier of S2. Stated more intuitively: if S1 decides whether or not S2 is 
reached, then S1 is a control dependency of S2. 

As a start, we add `Place.reactive: boolean` so that individual places can track 
whether they are reactive or not. This lets us do fine-grained reactivity 
inference on the control-flow graph, even taking into account different SSA 
instances of a variable, so that we can say that a particular SSA version of `x` 
is reactive, while other "versions" of x (due to reassignment) are not.

compiler/packages/babel-plugin-react-forget/src/HIR/BuildHIR.ts

@@ -76,6 +76,7 @@ export function lower(
       kind: "Identifier",
       identifier: builder.resolveBinding(ref),
       effect: Effect.Unknown,
+      reactive: false,
       loc: ref.loc ?? GeneratedSource,
     });
   }
@@ -106,6 +107,7 @@ export function lower(
         kind: "Identifier",
         identifier,
         effect: Effect.Unknown,
+        reactive: false,
         loc: param.node.loc ?? GeneratedSource,
       };
       params.push(place);
@@ -118,6 +120,7 @@ export function lower(
         kind: "Identifier",
         identifier: builder.makeTemporary(),
         effect: Effect.Unknown,
+        reactive: false,
         loc: param.node.loc ?? GeneratedSource,
       };
       params.push(place);
@@ -133,6 +136,7 @@ export function lower(
         kind: "Identifier",
         identifier: builder.makeTemporary(),
         effect: Effect.Unknown,
+        reactive: false,
         loc: param.node.loc ?? GeneratedSource,
       };
       params.push({
@@ -403,6 +407,7 @@ function lowerStatement(
             effect: Effect.Unknown,
             identifier,
             kind: "Identifier",
+            reactive: false,
             loc: id.node.loc ?? GeneratedSource,
           };
           lowerValueToTemporary(builder, {
@@ -790,6 +795,7 @@ function lowerStatement(
               effect: Effect.Unknown,
               identifier,
               kind: "Identifier",
+              reactive: false,
               loc: id.node.loc ?? GeneratedSource,
             };
             if (builder.isContextIdentifier(id)) {
@@ -1158,6 +1164,7 @@ function lowerStatement(
           kind: "Identifier",
           identifier: builder.makeTemporary(),
           effect: Effect.Unknown,
+          reactive: false,
           loc: handlerBindingPath.node.loc ?? GeneratedSource,
         };
         lowerValueToTemporary(builder, {
@@ -3021,6 +3028,7 @@ function lowerIdentifier(
     kind: "Identifier",
     identifier: identifier,
     effect: Effect.Unknown,
+    reactive: false,
     loc: exprLoc,
   };
   return place;
@@ -3034,6 +3042,7 @@ function buildTemporaryPlace(builder: HIRBuilder, loc: SourceLocation): Place {
     kind: "Identifier",
     identifier: builder.makeTemporary(),
     effect: Effect.Unknown,
+    reactive: false,
     loc,
   };
   return place;
@@ -3087,6 +3096,7 @@ function lowerIdentifierForAssignment(
     kind: "Identifier",
     identifier: identifier,
     effect: Effect.Unknown,
+    reactive: false,
     loc,
   };
   return place;

compiler/packages/babel-plugin-react-forget/src/HIR/HIR.ts

@@ -877,6 +877,7 @@ export type Place = {
   kind: "Identifier";
   identifier: Identifier;
   effect: Effect;
+  reactive: boolean;
   loc: SourceLocation;
 };
 

compiler/packages/babel-plugin-react-forget/src/HIR/MergeConsecutiveBlocks.ts

@@ -75,6 +75,7 @@ export function mergeConsecutiveBlocks(fn: HIRFunction): void {
           kind: "Identifier",
           identifier: phi.id,
           effect: Effect.ConditionallyMutate,
+          reactive: false,
           loc: GeneratedSource,
         },
         value: {
@@ -83,6 +84,7 @@ export function mergeConsecutiveBlocks(fn: HIRFunction): void {
             kind: "Identifier",
             identifier: operand,
             effect: Effect.Read,
+            reactive: false,
             loc: GeneratedSource,
           },
           loc: GeneratedSource,

compiler/packages/babel-plugin-react-forget/src/Inference/DropManualMemoization.ts

@@ -58,6 +58,7 @@ export function dropManualMemoization(func: HIRFunction): void {
                     kind: "Identifier",
                     identifier: fn.identifier,
                     effect: Effect.Unknown,
+                    reactive: false,
                     loc: instr.value.loc,
                   },
                   loc: instr.value.loc,

compiler/packages/babel-plugin-react-forget/src/Inference/InlineImmediatelyInvokedFunctionExpressions.ts

@@ -226,6 +226,7 @@ function rewriteBlock(
         type: makeType(),
       },
       kind: "Identifier",
+      reactive: false,
       loc: terminal.loc,
     },
     value: {
@@ -265,6 +266,7 @@ function declareTemporary(
         type: makeType(),
       },
       kind: "Identifier",
+      reactive: false,
       loc: GeneratedSource,
     },
     value: {