compiler: fix map optimization

Not all uses of a map are call instructions. Don't assume they are.
TODO: investigate these uses and see whether they might be eliminated?

compiler/optimizer.go

@@ -69,12 +69,16 @@ func (c *Compiler) OptimizeMaps() {
 		unknownUses := false // are there any uses other than setting a value?
 
 		for _, use := range getUses(makeInst) {
+			if use := use.IsACallInst(); !use.IsNil() {
 				switch use.CalledValue() {
 				case hashmapBinarySet, hashmapStringSet:
 					updateInsts = append(updateInsts, use)
 				default:
 					unknownUses = true
 				}
+			} else {
+				unknownUses = true
+			}
 		}
 
 		if !unknownUses {