I've been using this mode to narrow down llc unit tests. Example
custom compile script:
llc "$@"
not pygrep.py 'mul\s+r([0-9]), r\1,' < bugpoint-test-program.s
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125096 91177308-0d34-0410-b5e6-96231b3b80d8