[llvm] [Utils] update_llc_test_checks.py: updated the regexp for ARM target (PR #148287)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> fp16-promote.ll, cse-flags.ll: in these files there are some file_checks directives outside the function and this leads to an error since the update script rewrites the directives inside the function.

This is pretty common - the update script shouldn't touch comments outside functions so if the original CHECKS were put above the function then they won't get stripped and the lit test will fail as it ends up with duplicate checks.

https://github.com/llvm/llvm-project/pull/148287