[llvm] [AArch64] Add tests for redundant csel instructions. NFC (PR #101014)

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citymarina wrote:

1613781 is the followup that I plan to post after this.

Regarding the dead instructions, are you referring to the assignments to `%cmp1` and `%and.i`? I tried to remove these, but then the `csel` that I want to test is no longer produced (instead there is a `mvn` or `orn`).

I was suggested to include the `.ll` file as more of an end-to-end test, to guard against the possibility that maybe the bad `csel` might return at a later stage, which a narrow `-run-pass` test case wouldn't catch. However I am now starting to think that maybe this test is not enough of a controlled environment. If the test depends on having dead instructions, then that seems prone to breakage from random future changes. Do you have any advice on how to proceed?

https://github.com/llvm/llvm-project/pull/101014