Add a utility to update MIR checks, similar to update_llc_test_checks

This adds update_mir_test_checks, which updates the check lines in mir
tests. This can only update tests that start and end with .mir
currently (ie, -run-pass) but it should be sufficient for updating at
least some of the GlobalISel tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316057 91177308-0d34-0410-b5e6-96231b3b80d8