[llvm] update_test_checks: recognize %if in RUN line (PR #108972)

[Alexander Richardson via llvm-commits]

arichardson wrote:

It would be good to move any of this parsing to common so that the other update scripts also benfit from this (cc_test_checks, llc_test_checks, mir_test_checks).

I think the ideal solution would be to reuse lit code parse the RUN lines instead of re-implementing most of the syntax in these scripts. I started working on that a while ago but never got around to finishing it. 

https://github.com/llvm/llvm-project/pull/108972