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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334756 91177308-0d34-0410-b5e6-96231b3b80d8