X86, Win64: Allow 'mov' to restore the stack pointer if we have a FP

The Win64 epilogue structure is very restrictive, it permits a very
small number of opcodes and none of them are 'mov'.

This means that given:
  mov %rbp, %rsp
  pop %rbp

The mov isn't the epilogue, only the pop is.  This is problematic unless
a frame pointer is present in which case we are free to do whatever we'd
like in the "body" of the function.  If a frame pointer is present,
unwinding will undo the prologue operations in reverse order regardless
of the fact that we are at an instruction which is reseting the stack
pointer.

llvm-svn: 230543