[UpdateTestChecks] Prevent rapid onset insanity when forced to write...

[UpdateTestChecks] Prevent rapid onset insanity when forced to write LoopVectorize-driven costmodel tests

Subj, or on other words, we have a lot of tests that are driven by
the LoopVectorizer's debug output, but we don't have
any meaningful way to autogenerate checklines in them,
which means that an insurmountable amount of manual work
is required when modifying the appropriate cost models.

That is not sustainable, so this presents a solution.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D121133