[GlobalISel][X86] Support G_BRCOND operation.

Summary: Support G_BRCOND operation. For now don't try to fold cmp/trunc instructions.

Reviewers: zvi, guyblank

Reviewed By: guyblank

Subscribers: rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D34754

llvm-svn: 311327