[X86] Add BSR/BSF/BSWAP intrinsics to ia32intrin.h to match gcc.

Summary:
These are all implemented by icc as well.

I made bit_scan_forward/reverse forward to the __bsfd/__bsrq since we also have
__bsfq/__bsrq.

Note, when lzcnt is enabled the bsr intrinsics generates lzcnt+xor instead of bsr.

Reviewers: RKSimon, spatel

Subscribers: cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59682

llvm-svn: 356848