[GlobalISel][X86] G_LOAD/G_STORE pointer selection support.

Summary: [GlobalISel][X86] G_LOAD/G_STORE pointer selection support.

Reviewers: zvi, guyblank

Reviewed By: zvi, guyblank

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

Differential Revision: https://reviews.llvm.org/D32217

llvm-svn: 301788