[GlobalISel][X86] Support G_GLOBAL_VALUE operation.

Summary: Support G_GLOBAL_VALUE operation. For now most of the PIC configurations not implemented yet.

Reviewers: zvi, guyblank

Reviewed By: guyblank

Subscribers: rovka, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D34738

Conflicts:
	test/CodeGen/X86/GlobalISel/regbankselect-X86_64.mir

llvm-svn: 306972