[5.7] Fix infinite loop and scalar matching in grapheme mode + scalar matching optimizations#569

hamishknight merged 2 commits intoswiftlang:swift/release/5.7swiftlang/swift-experimental-string-processing:swift/release/5.7from

rctcwyvrn:uhoh-infinite-loop-5.7rctcwyvrn/swift-experimental-string-processing:uhoh-infinite-loop-5.7Copy head branch name to clipboard