Unbundle KILL bundles in VirtRegRewriter

SplitKit forms invalid COPY subreg bundles without a leading
BUNDLE instruction. That manifests itself in post-RA scheduler
counting instruction and asserting on "Instruction count mismatch".

The bundle shall be undone by VirtRegRewriter::expandCopyBundle(),
but it does not because VirtRegRewriter::handleIdentityCopy() can
turn COPY bundle into a KILL bundle.

Process KILLs as well.

Differential Revision: https://reviews.llvm.org/D85484