ExpandPostRAPseudos should transfer implicit uses, not only implicit defs

ExpandPostRAPseudos should transfer implicit uses, not only implicit defs

Previously, we would expand:
%BL<def> = COPY %DL<kill>, %EBX<imp-use,kill>, %EBX<imp-def>
Into:
%BL<def> = MOV8rr %DL<kill>, %EBX<imp-def>
Dropping the imp-use on the floor.

That confused CriticalAntiDepBreaker, which (correctly) assumes that if an
instruction defs but doesn't use a register, that register is dead immediately
before the instruction - while in this case, the high lanes of EBX can be very
much alive.

This fixes PR28560.

Differential Revision: https://reviews.llvm.org/D22425

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275634 91177308-0d34-0410-b5e6-96231b3b80d8