[llvm] [GlobalISel] Add `G_EXTRACT_VECTOR_ELT` for `computeKnownBits` (PR #164825)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

The use of umull looks nice.

I'm not sure if in GISel the return type can be different from the vector element type, as we can be less strict with the types. There is a chance we still use that for selecting patterns.

Can we either add a test for the case of extending return type, or bail out in that situation?

https://github.com/llvm/llvm-project/pull/164825