Fix issues with LLDB's interpreter and MS ABI guard variables.

MS ABI guard variables end with @4IA, so this patch teaches the
interpreter about that.  Additionally, there was an issue with
TurnGuardLoadIntoZero which was causing some guard uses of a
variable to be missed.  This fixes that by calling
Instruction::replaceAllUsesWith() instead of trying to replicate
that function.

llvm-svn: 225547