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

[Moritz Zielke via llvm-commits]

mooori wrote:

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

I've gone with bailing out in 9a8b6ee. It assumes that at the level of `LLT` and `EXTRACT_VECTOR_ELEMENT`, comparing the bit widths of the element type and return type is sufficient to detect `return type != element type`.

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