MachineInstr: Respect register aliases in clearRegiserKills()

This fixes bugs in copy elimination code in llvm. It slightly changes the
semantics of clearRegisterKills(). This is appropriate because:
- Users in lib/CodeGen/MachineCopyPropagation.cpp and
  lib/Target/AArch64RedundantCopyElimination.cpp and
  lib/Target/SystemZ/SystemZElimCompare.cpp are incorrect without it
  (see included testcase).
- All other users in llvm are unaffected (they pass TRI==nullptr)
- (Kill flags are optional anyway so removing too many shouldn't hurt.)

Differential Revision: http://reviews.llvm.org/D17554

llvm-svn: 261763