[GlobalISel][X86] Support vector type G_MERGE_VALUES selection.

Summary:
Support vector type G_MERGE_VALUES selection. For now G_MERGE_VALUES marked as legal for any type, so nothing to do in legalizer.
Split from https://reviews.llvm.org/D33665

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

Reviewed By: guyblank

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

Differential Revision: https://reviews.llvm.org/D33958

llvm-svn: 306665