[GlobalISel][X86] Fold FI/G_GEP into LDR/STR instruction addressing mode.

Summary: Implement some of the simplest addressing modes.It should help to test ABI.

Reviewers: zvi, guyblank

Reviewed By: guyblank

Subscribers: rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D33888

llvm-svn: 305691