[llvm] [GlobalIsel][AArch64] Replace N bit G_ADD with N/2 bit G_ADD if the lower bits are known to be zeros (PR #101327)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

We have both combines independent of pre- or post-legalization. I outlined a harsh combine for G_UADDE in 
https://github.com/llvm/llvm-project/pull/97194 , but I didn't had the bandwidth yet.

https://github.com/llvm/llvm-project/pull/101327