[CodeGen] Fix problem with X86 byte registers in CriticalAntiDepBreaker

CriticalAntiDepBreaker was not correctly tracking defs of the high X86 byte
registers, leading to incorrect use of a busy register to break an
antidependence.

Fixes pr27681, and its duplicates pr27580, pr27804.

Differential Revision: http://reviews.llvm.org/D20456

llvm-svn: 270935