[UpdateTestChecks] Auto-generate stub bodies for unused prefixes

This is scoped to autogenerated tests.

The goal is to support having each RUN line specify a list of
check-prefixes where one can specify potentially redundant prefixes. For example,
for X86, if one specified prefixes for  both AVX1 and AVX2, and the codegen happened to
match today, one of the prefixes would be used and the onther one not.
If the unused prefix were dropped, and later, codegen differences were
introduced, one would have to go figure out where to add what prefix
(paraphrasing
https://lists.llvm.org/pipermail/llvm-dev/2021-February/148326.html)

To avoid getting errors due to unused prefixes, whole directories can be
opted out (as discussed on that thread), but that means that tests that
aren't autogenerated in such directories could have undetected unused
prefix bugs.

This patch proposes an alternative that both avoids the above, dir-level
optout, and supports the main autogen scenario discussed first. The autogen
tool appen...