The memrchr enhancement added in D123629 has a logic bug that renders it ineffective in a subset of instances. The bug is (most likely) assuming that the second argument to StringRef::find() is the number of characters to search rather than the offset to search from. The tests that were added for these instances were precommitted so they didn't fail and during review none of us noticed the expected folding wasn't done.(*)
This change corrects the mistake and enables the folding.
(*) This seems like another gotcha to keep in mind with precommitting tests, one that could be helped by having XFAIL directives for individual assertions.)