[RegisterCoalescer] Make sure each live-range has only one component, as

demanded by the machine verifier.
After shrinking a live-range to its uses, it is possible to create several
smaller live-ranges. When this happens, shrinkToUses returns true and we need to
split the different components into their own live-ranges.

The problem does not reproduce on any in-tree target but Jonas Paulsson
<jonas.paulsson@ericsson.com>, who reported the problem, checked that this patch
fixes the issue.

llvm-svn: 236658