[X86] Add notes to a few intrinsics

This a change corresponding to the clang change in
https://reviews.llvm.org/D45616

Reviewers: craig.topper, uriel.k, RKSimon, andrew.w.kaylor, spatel, scanon, efriedma

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D48280

llvm-svn: 335340