Support other llc-like tools in update_llc_test_checks.py

Summary:
If you have downstream tools that are llc-like (e.g, llc with different
defaults), it's convenient to still be able to use
`update_llc_test_checks` with them. Refactor slightly to allow such tools
to be supported by adding them to LLC_LIKE_TOOLS

Reviewers: bogner

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80960