[llvm] [GlobalIsel] Combine G_EXTRACT_VECTOR_ELT (PR #85321)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

There is precedence in InstCombine.
https://github.com/llvm/llvm-project/tree/main/llvm/lib/Transforms/InstCombine
I could also envision CombinerHelperInteger, CombinerHelperPhi,  ...
The motivation was for modularity and when the combiner grows, we can still use GitHub for git blame.

I also want to add combines for at least G_INSERT_VECTOR_ELT, G_INSERT_SUBVECTOR, G_EXTRACT_SUBVECTOR.

https://github.com/llvm/llvm-project/pull/85321