x86 FP atomic codegen: don't drop globals, stack

Summary:
x86 codegen is clever about generating good code for relaxed
floating-point operations, but it was being silly when globals and
immediates were involved, forgetting where the global was and
loading/storing from/to the wrong place. The same applied to hard-coded
address immediates.

Don't let it forget about the displacement.

This fixes https://llvm.org/bugs/show_bug.cgi?id=25171

A very similar bug when doing floating-points atomics to the stack is
also fixed by this patch.

This fixes https://llvm.org/bugs/show_bug.cgi?id=25144

Reviewers: pete

Subscribers: llvm-commits, majnemer, rsmith

Differential Revision: http://reviews.llvm.org/D13749

llvm-svn: 250429