x87StackOptimizationPass: Fix isValid()

Previously this wouldn't have worked, since .first isn't a valid member.
The only reason it wasn't caught is because the function is never
instantiated.

FEXCore/Source/Interface/IR/Passes/x87StackOptimizationPass.cpp

@@ -96,7 +96,7 @@ public:
   }
 
   bool isValid(size_t Offset) const {
-    return buffer[Offset].first;
+    return buffer[Offset].Type == StackSlot::VALID;
   }
 
   void clear() {