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Summary: After their range checks were removed in 7f50c15be5c0, br_tables started being duplicated into their predecessors by tail folding. Unfortunately, when the br_tables were in loops this transformation introduced bad irreducible control flow which was later expanded into even more br_tables. This commit abuses the `isNotDuplicable` property to prevent this irreducible control flow from being introduced. This change saves a few dozen bytes of code size and has a negligible affect on performance for most of the large Emscripten benchmarks, but can improve performance significantly on microbenchmarks of switches in loops. Reviewers: aheejin, dschuff Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D81628
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