x86: preserve flags when folding atomic operations

Summary:
D4796 taught LLVM to fold some atomic integer operations into a single
instruction. The pattern was unaware that the instructions clobbered
flags.

This patch adds the missing EFLAGS definition.

Floating point operations don't set flags, the subsequent fadd
optimization is therefore correct. The same applies for surrounding
load/store optimizations.

Reviewers: rsmith, rtrieu

Subscribers: llvm-commits, reames, morisset

Differential Revision: http://reviews.llvm.org/D13680

llvm-svn: 250135