[VirtRegRewriter] Avoid clobbering registers when expanding copy bundles

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.

Differential Revision: https://reviews.llvm.org/D48154

llvm-svn: 334750