[RegisterCoalescer] Fix for assert in removePartialRedundancy

Summary:
I got "Use not jointly dominated by defs" when removePartialRedundancy
attempted to prune then re-extend a subrange whose only liveness was a
dead def at the copy being removed.

V2: Removed junk from test. Improved comment.
V3: Addressed minor review comments.

Subscribers: MatzeB, qcolombet, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D50914

Change-Id: I6f894e9f517f71e921e0c6d81d28c5f344db8dad
llvm-svn: 340549