[llvm] [GlobalISel] Add support to moreElementsVector for G_SEXT, G_ZEXT and G_ANYEXT (PR #85038)

[via llvm-commits]

chuongg3 wrote:

Would it be possible to replace `.maxNumElements()` with `.clampNumElements()`?
Currently moreElements only gets called by `.moreElementsToNextPow2()` for the EXTEND instructions
This might help with legalizing quite a few of the failing EXTEND instructions that has been showing up like:
`%1:_(<4 x s16>) = G_ZEXT %0:_(<4 x s8>)`

https://github.com/llvm/llvm-project/pull/85038