[llvm] [RISCV][ISel] Fold trunc (lshr (add (zext X), (zext Y)), 1) -> vaaddu X, Y (PR #76550)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

Just a couple of nits but otherwise LGTM, thanks! Should probably wait for a second approval too in case anyone has other opinions 

https://github.com/llvm/llvm-project/pull/76550