[GlobalISel][X86] G_MUL legalizer/selector support.

Summary:
G_MUL legalizer/selector/regbank support.
Use only Tablegen-erated instruction selection.
This patch dealing with legal operations only.

Reviewers: zvi, guyblank

Reviewed By: guyblank

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

Differential Revision: https://reviews.llvm.org/D32698

llvm-svn: 302410