[llvm] [InstCombine] Fold `(op x, ({z,s}ext (icmp eq x, C)))` to select (PR #89020)

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goldsteinn wrote:

> Reverse ping.

Was waiting for #88579 to get in before rebasing this.

I can rebase/split shl stuff if you want it to proceed concurrently though.

https://github.com/llvm/llvm-project/pull/89020