[X86FixupBWInsts] Fix miscompilation if sibling sub-register is live.

Summary: The issues was found during D40524.

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

Reviewed By: andrew.w.kaylor

Subscribers: aivchenk, llvm-commits

Differential Revision: https://reviews.llvm.org/D42533

llvm-svn: 323635