[llvm] [InstCombine, AArch64] Avoid vector Ext in case by-element operation variant apply for all elements (PR #140733)

[David Green via llvm-commits]

davemgreen wrote:

>From what I remember this was intended to promote more generation of smull/umull instructions. It is probably worth making sure that keeps happening, especially for v2i64 types where the mul would otherwise be scalarized.

https://github.com/llvm/llvm-project/pull/140733