[llvm] [AArch64][GlobalISel] Select UMULL instruction (PR #65469)

[Amara Emerson via llvm-commits]

aemerson wrote:

As long as PostLegalizerLowering scalarizes it I think it's fine. That pass runs even at -O0 so a v2s64 MUL should never survive past that.

https://github.com/llvm/llvm-project/pull/65469