More fixes for subreg join failure in RegCoalescer

Summary:
Part of the adjustCopiesBackFrom method wasn't correctly dealing with SubRange
intervals when updating.

2 changes. The first to ensure that bogus SubRange Segments aren't propagated when
encountering Segments of the form [1234r, 1234d:0) when preparing to merge value
numbers. These can be removed in this case.

The second forces a shrinkToUses call if SubRanges end on the copy index
(instead of just the parent register).

V2: Addressed review comments, plus MIR test instead of ll test

Subscribers: MatzeB, qcolombet, nhaehnle

Differential Revision: https://reviews.llvm.org/D40308

Change-Id: I1d2b2b4beea802fce11da01edf71feb2064aab05
llvm-svn: 337273