update_mir_test_checks: Do a better job of disambiguating names

Matching prefixes isn't good enough, because it leads to things like
calling the first constant C3 just because there were two copies
before it. Tighten up the check to match more precisely, but also be
careful about ambiguity when dealing with target opcodes that end in a
number.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316088 91177308-0d34-0410-b5e6-96231b3b80d8