[test] Fix bigint-int64-lowered test in GC stress mode

The flag combination --gc-interval=500 --stress-compaction
--stress-flush-bytecode, combined with baking mjsunit.js into the
custom snapshot, caused type feedback for "deepEquals" to be
forgotten, leading to an unexpected soft deopt. Forcing type feedback
collection with %PrepareFunctionForOptimization() fixes that.

Change-Id: I954c7ecbe70ca5b803a5fa7cd809c118f7659f21
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1946347Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65281}

test/mjsunit/compiler/bigint-int64-lowered.js

@@ -75,6 +75,7 @@ function TestInt64LoweredOperations() {
 function OptimizeAndTest(fn) {
   %PrepareFunctionForOptimization(fn);
   %PrepareFunctionForOptimization(assertEquals);
+  %PrepareFunctionForOptimization(deepEquals);
   fn();
   fn();
   %OptimizeFunctionOnNextCall(fn);