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
