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

Summary:
Support vector type G_UNMERGE_VALUES selection.
For now G_UNMERGE_VALUES marked as legal for any type, so nothing to do in legalizer.

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

Reviewed By: guyblank

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

Differential Revision: https://reviews.llvm.org/D33665

llvm-svn: 306971