[X86] Don't report gather is legal on Skylake CPUs when AVX2/AVX512 is...

[X86] Don't report gather is legal on Skylake CPUs when AVX2/AVX512 is disabled. Allow gather on SKX/CNL/ICL when AVX512 is disabled by using AVX2 instructions.

Summary:
This adds a new fast gather feature bit to cover all CPUs that support fast gather that we can use independent of whether the AVX512 feature is enabled. I'm only using this new bit to qualify AVX2 codegen. AVX512 is still implicitly assuming fast gather to keep tests working and to match the scatter behavior.

Test command lines have been added for these two cases.

Reviewers: magabari, delena, RKSimon, zvi

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40282

llvm-svn: 318983