update_mir_test_checks: Better handling of common prefixes

Support the pattern where a test file uses multiple prefixes per run line:
one prefix that is unique to the run line, and additional prefixes that are
common with other run lines.

Decide on a per-function basis which prefix(es) to emit, based on which run
lines have the same output.

Move the renaming of vregs earlier, so that we can compare the output as it
would actually be printed in check lines.

Differential Revision: https://reviews.llvm.org/D126411