X86: Don't emit SAHF/LAHF for 64-bit targets unless explicitly supported

X86: Don't emit SAHF/LAHF for 64-bit targets unless explicitly supported

These instructions are not supported by all CPUs in 64-bit mode. Emitting them
causes Chromium to crash on start-up for users with such chips.

(GCC puts these instructions behind -msahf on 64-bit for the same reason.)

This patch adds FeatureLAHFSAHF, enables it by default for 32-bit targets
and modern CPUs, and changes X86InstrInfo::copyPhysReg back to the lowering
from before r244503 when the instructions are not available.

Differential Revision: http://reviews.llvm.org/D15240

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254793 91177308-0d34-0410-b5e6-96231b3b80d8