[GlobalISel][X86] Get correct RegClass for given RegBank.

Summary:
In some cases RegClass depends on target feature. Hight (16-31) vector registers exist only if AVX512f available.
Split from https://reviews.llvm.org/D33665

Reviewers: qcolombet, t.p.northover, zvi, guyblank

Reviewed By: t.p.northover, guyblank

Subscribers: guyblank, rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D33952

Conflicts:
	test/CodeGen/X86/GlobalISel/select-memop-scalar.mir

llvm-svn: 305784