[clang-tools-extra] [InstCombine] Convert or concat to fshl if opposite or concat exists (PR #68502)

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goldsteinn wrote:

See most recent comments about missing tests. That being said code looks functionally correct to me.
Still not 100% sure this is a desirable change. Will defer to @nikic about that.

https://github.com/llvm/llvm-project/pull/68502