[vm] Fix poor hash performance of CodeSet.

Observed this taking ~2% of debug gen_snapshot time.

TEST=ci
Change-Id: I657148904fe2b5a8f3dfbd006c3d1327aa3866ef
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506261
Reviewed-by: Tess Strickland <sstrickl@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/compiler/aot/precompiler.cc

@@ -3278,7 +3278,10 @@ void Precompiler::PruneDictionaries() {
 
 // Traits for the HashTable template.
 struct CodeKeyTraits {
-  static uint32_t Hash(const Object& key) { return Code::Cast(key).Size(); }
+  static uint32_t Hash(const Object& key) {
+    // Instructions never move.
+    return FinalizeHash(Code::Cast(key).PayloadStart());
+  }
   static const char* Name() { return "CodeKeyTraits"; }
   static bool IsMatch(const Object& x, const Object& y) {
     return x.ptr() == y.ptr();