[llvm] [GlobalISel] Refactor extractParts() (PR #75223)

[via llvm-commits]

qcolombet wrote:

> Moved extractParts() and extractVectorParts() from LegalizerHelper to Utils to be able to use it in different passes.
>

Could you elaborate on which passes may want to use this functionality while not using the `LegalizerHelper` directly?

https://github.com/llvm/llvm-project/pull/75223