Reviewed by Gavin Barraclough.

        Add some aggressive GC validation to debug builds.
        https://bugs.webkit.org/show_bug.cgi?id=60601

        When assertions are enabled we now do some validity checking
        of objects being added to the mark stack.

        * bytecode/Instruction.h:
        (JSC::PolymorphicAccessStructureList::PolymorphicStubInfo::PolymorphicStubInfo):
        (JSC::PolymorphicAccessStructureList::visitAggregate):
        * heap/MarkStack.cpp:
        (JSC::MarkStack::validateSet):
        (JSC::MarkStack::validateValue):
        * heap/MarkStack.h:
        (JSC::MarkStack::appendValues):
        (JSC::MarkStack::append):
        (JSC::MarkStack::internalAppend):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@86202 268f45cc-cd09-0410-ab3c-d52691b4dbfc