[RegisterCoalescer] Fix for SubRange join unreachable

Summary:
During remat, some subranges might end up having invalid segments which caused problems for later
coalescing.

Added in a check to remove segments that are invalidated as part of the remat.

See http://llvm.org/PR33524

Subscribers: MatzeB, qcolombet

Differential Revision: https://reviews.llvm.org/D34391

llvm-svn: 307247