[X86, Win64] Use a frame pointer if pushf is emitted

A frame pointer must be used if stack pointer is modified after the
prologue.  LLVM will emit pushf/popf if we need to save/restore the
FLAGS register, requiring us to have a frame pointer for the function.

There is a small twist: this sequence might exist in user code via
inline-assembly.  For now, conservatively assume that such functions
require a frame pointer.  For real world justification, please see
clang's implementation of __readeflags.

This fixes PR25945.

llvm-svn: 256456