[X86] Add the movbe instruction intrinsics from icc.

These intrinsics exist in icc. They can be found on the Intel Intrinsics Guide website.

All the backend support is in place to pattern match a load+bswap or a bswap+store pattern to the MOVBE instructions. So we just need to get the frontend to emit the correct IR. The pointer arguments in icc are declared as void so I had to jump through a packed struct to forcing a specific alignment on the load/store. Same trick we use in the unaligned vector load/store intrinsics

Differential Revision: https://reviews.llvm.org/D52586

llvm-svn: 343343