[llvm] [SimplifyCFG] Simplify switch with implicit default (PR #95665)

[via llvm-commits]

YanWQ-monad wrote:

I looked into the regressions in https://github.com/dtcxzyw/llvm-opt-benchmark/pull/691#discussion_r1641855087, I don't have an idea about how to fix it. It's originally folded by both `simplifyBranch` and `TurnSwitchRangeIntoICmp`, but this PR breaks the pattern before `simplifyBranch` runs.
   I don't know if it's worth adding an option like `Options.SimplifySwitchWithImplicitDefault` to disable it in the early pipeline, as this fold may break the continuity of a switch, which may interfere with other folds.

https://github.com/llvm/llvm-project/pull/95665