[llvm] [AArch64][GlobalISel] Adopt some Ld* patterns to reduce codegen regressions (PR #135492)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> v16i8 in SelectionDAG expects i32 scalar argument type, whereas G_INSERT_VECTOR_ELT expects s8.

Should fix the legalization behavior instead to fix the index type? Is there some inherent reason to prefer the illegal register typed index? 

https://github.com/llvm/llvm-project/pull/135492