Re-apply "[VirtRegRewriter] Avoid clobbering registers when expanding copy bundles"

This is r334750 (which was reverted in r334754) with a fix for an
uninitialized variable that was caught by msan.

Original commit message:
> If a copy bundle happens to involve overlapping registers, we can end
> up with emitting the copies in an order that ends up clobbering some
> of the subregisters. Since instructions in the copy bundle
> semantically happen at the same time, this is incorrect and we need to
> make sure we order the copies such that this doesn't happen.

llvm-svn: 334756