More precise error for canonical reassignment in a value block

These examples previously errored all the way in codegen, when we detected that 
a value block (eg a `while` test expression) was declaring a new variable. We 
now detect this in LeaveSSA and error. The actual fix is a bit tricky, we'd need 
to add a new declaration in the nearest block scope (or selectively not DCE the 
declaration if its reassigned in just this way).

compiler/forget/src/SSA/LeaveSSA.ts

@@ -137,6 +137,12 @@ export function leaveSSA(fn: HIRFunction): void {
             originalLVal === undefined ||
             originalLVal.lvalue === value.lvalue // in case this was pre-declared for the `for` initializer
           ) {
+            if (originalLVal === undefined && block.kind !== "block") {
+              CompilerError.invariant(
+                `TODO: Handle reassignment in a value block where the original declaration was removed by dead code elimination (DCE)`,
+                value.lvalue.place.loc
+              );
+            }
             declarations.set(value.lvalue.place.identifier.name, {
               lvalue: value.lvalue,
               place: value.lvalue.place,
@@ -170,6 +176,12 @@ export function leaveSSA(fn: HIRFunction): void {
               originalLVal === undefined ||
               originalLVal.lvalue === value.lvalue
             ) {
+              if (originalLVal === undefined && block.kind !== "block") {
+                CompilerError.invariant(
+                  `TODO: Handle reassignment in a value block where the original declaration was removed by dead code elimination (DCE)`,
+                  place.loc
+                );
+              }
               declarations.set(place.identifier.name, {
                 lvalue: value.lvalue,
                 place,

compiler/forget/src/__tests__/fixtures/compiler/error.while-with-assignment-in-test.expect.md

@@ -19,7 +19,7 @@ function f(reader) {
 ## Error
 
 ```
-[ReactForget] Todo: (CodegenReactiveFunction::codegenInstructionValue) Cannot declare variables in a value block, tried to declare 'value'
+[ReactForget] Invariant: TODO: Handle reassignment in a value block where the original declaration was removed by dead code elimination (DCE) (6:6)
 ```