[AVX-512] Make VBMI instruction set enabling imply that the BWI instruction set is also enabled.

Summary: This is needed to make the v64i8 and v32i16 types legal for the 512-bit VBMI instructions. Fixes PR30912.

Reviewers: delena, zvi

Subscribers: RKSimon, cfe-commits

Differential Revision: https://reviews.llvm.org/D26306

llvm-svn: 286340