[llvm] [AArch64][GlobalISel] Don't crash when legalising vector G_SHL (PR #168848)

[David Green via llvm-commits]

davemgreen wrote:

Im not sure if the combine we have for trunc(shift) -> shift(trunk) has been very useful and has led to a number of issues, both correctness and performance.

But I think with how we have it at the moment, the `moreElementsVectorSrc(MI, MoreTy, 2);` line is incorrect and needs to adjust the type it uses so that it has the same number of lanes as MoreTy but the elements of the existing type. The casts need to do work in the same way.

https://github.com/llvm/llvm-project/pull/168848