[test] Add -verify-coalescing to testcase and fix problems

Apparently the testcase
 coalesce-partial-redundant-reguse-terminator.mir
was broken in a way that -verify-coalescing detected.

Update the testcase so -verify-coalescing doesn't complain and so
that it still exposes the problem originally fixed in 6c062b76.

Differential Revision: https://reviews.llvm.org/D158397