Fix compilation with gcc

Currently getting the following error with gcc 8.4,
including on x64 linux:
```
error: ':InterpreterState::scratch_' is used uninitialized in this function
```

Change-Id: I95ae848bf2503f6a0dac30254b19b08047b73cce
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3683104Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#80901}

test/cctest/compiler/test-gap-resolver.cc

@@ -209,7 +209,7 @@ class InterpreterState {
   }
 
   OperandMap values_;
-  Key scratch_;
+  Key scratch_ = {};
 };
 
 // An abstract interpreter for moves, swaps and parallel moves.