[be] Clarify type of value block final value before change

Makes the type of the final value of a ValueBlock more precise, prior to 
changing it

compiler/forget/src/HIR/HIR.ts

@@ -63,7 +63,7 @@ export type ReactiveValueBlock = {
   kind: "value-block";
   instructions: ReactiveBlock;
   last: {
-    value: InstructionValue;
+    value: Place;
     id: InstructionId;
   } | null;
 };

compiler/forget/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -418,7 +418,7 @@ class Driver {
   visitValueBlock(
     parent: ReactiveBlock,
     block: BasicBlock,
-    terminalValue?: { value: InstructionValue; id: InstructionId }
+    terminalValue?: { value: Place; id: InstructionId }
   ): ReactiveValueBlock {
     const valueBlock: ReactiveValueBlock = {
       kind: "value-block",