[X86] Enable isel to use the PAUSE instruction even when SSE2 is disabled. Clang...

[X86] Enable isel to use the PAUSE instruction even when SSE2 is disabled. Clang part

Summary:
On older processors this instruction encoding is treated as a NOP.

MSVC doesn't disable intrinsics based on features the way clang/gcc does. Because the PAUSE instruction encoding doesn't crash older processors, some software out there uses these intrinsics without checking for SSE2.

This change also seems to also be consistent with gcc behavior.

Fixes PR34079

Reviewers: RKSimon, zvi

Reviewed By: RKSimon

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36362

llvm-svn: 310191