[X86] Remove some preprocessor feature checks from intrinsic headers

Summary:
These look to be a couple things that weren't removed when we switched to target attribute.

The popcnt makes including just smmintrin.h also include popcntintrin.h. The popcnt file itself already contains target attrributes.

The prefetch ones are just wrappers around __builtin_prefetch which we have graceful fallbacks for in the backend if the exact instruction isn't available. So there's no reason to hide them. And it makes them available in functions that have the write target attribute but not a -march command line flag.

Reviewers: echristo, RKSimon, spatel, DavidKreitzer

Reviewed By: echristo

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47029

llvm-svn: 332830