[X86FixupBWInsts] More precise register liveness if no <imp-use> on MOVs.

Summary:
Subregister liveness tracking is not implemented for X86 backend, so
sometimes the whole super register is said to be live, when only a
subregister is really live. That might happen if the def and the use
are located in different MBBs, see added fixup-bw-isnt.mir test.

However, using knowledge of the specific instructions handled by the
bw-fixup-pass we can get more precise liveness information which this
change does.

Reviewers: MatzeB, DavidKreitzer, ab, andrew.w.kaylor, craig.topper

Reviewed By: craig.topper

Subscribers: n.bozhenov, myatsina, llvm-commits, hiraditya

Patch by Andrei Elovikov <andrei.elovikov@intel.com>

Differential Revision: https://reviews.llvm.org/D37559

llvm-svn: 313524