update_llc_test_checks: better support for non-llc tools (#93135)

A full downstream fork can already hack up update_llc_test_checks.py to
support custom tools that output assembly.

An out-of-tree frontend which is meant to build against upstream
llvm-project cannot do this, and so providing additional arguments to
support a non-standard tool is useful.

This also makes a minor adjustment to the regular expression for
matching AMDGPU functions when fewer comments are enabled, which happens
to be the case for our out-of-tree shader compiler (which motivated this
change).