Fix addRegisterDead and addRegisterKilled to be more thorough

when searching for redundant subregister dead/kill bits.

Previously it was common to see instructions marked like this:
  "RET %EAX<imp-use,kill>, %AX<imp-use,kill>"

With this change, addRegisterKilled continues scanning after
finding the %EAX operand, so it proceeds to discover the
redundant %AX kill and eliminates it, producing this:
  "RET %EAX<imp-use,kill>"

This currently has no effect on the generated code.

llvm-svn: 55698