[GlobalISel] Fix insertion of stack-protector epilogue
- Tell the StackProtector pass to generate the epilogue instrumentation when GlobalISel is enabled because GISel currently does not implement the same deferred epilogue insertion as SelectionDAG.
- Update StackProtector::InsertStackProtectors() to find a stack guard slot by searching for the llvm.stackprotector intrinsic when the prologue was not created by StackProtector itself but the pass still needs to generate the epilogue instrumentation. This fixes a problem when the pass would abort because the stack guard AllocInst pointer was null when generating the epilogue -- test CodeGen/AArch64/GlobalISel/arm64-irtranslator-stackprotect.ll.
Differential Revision: https://reviews.llvm.org/D54518