[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

llvm-svn: 347862