[GlobalISel][X86] G_ICMP support.

Summary: support G_ICMP for scalar types i8/i16/i64.

Reviewers: zvi, guyblank

Reviewed By: guyblank

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

Differential Revision: https://reviews.llvm.org/D32995

llvm-svn: 302774