Check for stale files in src/test/mir-opt #97564
Description
This folder currently contains a number of inst_combine_deref*.diff files that are completely unused - the associated .rs file that generated them was removed all the way back in #81101 . In addition to deleting these files, we should also add a check to compiletest that makes sure that the mir-opt folder contains no files that do not belong to any tests (with the exception of the readme).
@rustbot label +C-enhancement +A-test-suite +A-mir-opt