[vm/switchable_call] Refactor SwitchableCallHandler so that resolution is done outside of RunWithStoppedMutators.

The only exception is DoMonomorphicMiss which still resolves "old_target" inside RunWithStoppedMutators.

Bug: #36097

Change-Id: I5a199113d8ff3cc9064860b8e385f65642bd740c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150200
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>

Author: aam

runtime/vm/object.h

@@ -6515,7 +6515,7 @@ class Code : public Object {
   friend class MegamorphicCacheTable;  // for set_object_pool
   friend class CodePatcher;            // for set_instructions
   friend class ProgramVisitor;         // for set_instructions
-  // So that the RawFunction pointer visitor can determine whether code the
+  // So that the FunctionLayout pointer visitor can determine whether code the
   // function points to is optimized.
   friend class FunctionLayout;
   friend class CallSiteResetter;