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FileCheck currently compiles a regular expression of the form `Prefix1|Prefix2|...` and uses it to find the next prefix in the input. If we had a fast regex implementation, this would be a useful thing to do, as the regex implementation would be able to match multiple prefixes more efficiently than a naive approach. However, with our actual regex implementation, finding the prefixes basically becomes O(InputLen * RegexLen * LargeConstantFactor), which is a lot worse than a simple string search. Replace the regex with StringRef::find(), and keeping track of the next position of each prefix. There are various ways this could be improved on, but it's already significantly faster that the previous approach. For me, this improves check-llvm time from 138.5s to 132.5s, so by around 4-5%. For vector-interleaved-load-i16-stride-7.ll in particular, test time drops from 5s to 2.5s.