[llvm] [VectorCombine] Add a cost model for shuffleToIdentity. (PR #93937)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

I am hoping this won't be needed, as the regression from #92794 has been fixed in the backend. It might depend upon what happens in #93543. And I have updated the code with a few changes and some modifications to the tests in case it is.

The other thing I was looking at (which turned out to be piece-wise identities, not splats) can check the costs it needs elsewhere, and the difficult part is trying to make sure there is a valid place to insert the new instruction.

https://github.com/llvm/llvm-project/pull/93937