[LV][X86] Support of AVX2 Gathers code generation and update the LV with this

This patch depends on: https://reviews.llvm.org/D35348

Support of pattern selection of masked gathers of AVX2 (X86\AVX2 code gen)
Update LoopVectorize to generate gathers for AVX2 processors.

Reviewers: delena, zvi, RKSimon, craig.topper, aaboud, igorb

Reviewed By: delena, RKSimon

Differential Revision: https://reviews.llvm.org/D35772

llvm-svn: 318641