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

[Dhruv Chawla via llvm-commits]

dc03-work wrote:

> Would it be possible to replace `.maxNumElements()` with `.clampNumElements()` in AArch64LegalizerInfo.cpp? 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>)`

Hmm, what would the lower bound be in this case? 

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