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

Summary:
Support vector type G_EXTRACT selection. For now G_EXTRACT 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: guyblank, rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D33957

llvm-svn: 306240