[GlobalIsel][X86] support G_CONSTANT selection.

Summary: [GlobalISel][X86] support G_CONSTANT selection. Add regbank select tests.

Reviewers: zvi, guyblank

Reviewed By: guyblank

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

Differential Revision: https://reviews.llvm.org/D31974

llvm-svn: 300057