Fix mapInstructionOperands() visitor

Found while debugging the previous issue: `mapInstructionOperands()` should not 
look at lvalues. The previous version was causing us to create extra phi nodes, 
which interestingly weren't the actual problem behind the "SSA" bug, but sure 
looked like it at first.

compiler/forget/src/HIR/visitors.ts

@@ -293,12 +293,10 @@ export function mapInstructionOperands(
       break;
     }
     case "StoreLocal": {
-      instrValue.lvalue.place = fn(instrValue.lvalue.place);
       instrValue.value = fn(instrValue.value);
       break;
     }
     case "Destructure": {
-      mapPatternOperands(instrValue.lvalue.pattern, fn);
       instrValue.value = fn(instrValue.value);
       break;
     }