[clang] [llvm] [DropUnnecessaryAssumes] Add pass for dropping assumes (PR #159403)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw approved this pull request.

LG. Thanks.
FYI I did a similar attempt last year to drop useless assumptions (inserted by rustc): https://github.com/llvm/llvm-project/compare/main...dtcxzyw:llvm-project:perf/remove-dead-assume. It removes assumptions in leaf nodes of the dominator tree. Unfortunately it didn't work because SimplifyCFG may remove other edges and make them useful again.

https://github.com/llvm/llvm-project/pull/159403