Fix RegScavenger::unprocess

RegScavenger::unprocess walks backward, so it should undo the effects
of defs before undoing effects of kills. Previously it did things in
the opposite order, leaving a register apparently unused (dead) in the
case where an instruction both used (killed) and defined a register.

Differential Revision: https://reviews.llvm.org/D42200

llvm-svn: 337735