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

[Luke Lau via llvm-commits]

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lukel97 wrote:

Can we add some tests for vaaddu.vx? Something like

```llvm
define <vscale x 8 x i8> @vaaddu_vx_nxv8i8(<vscale x 8 x i8> %x, i8 %y) {
  %xzv = zext <vscale x 8 x i8> %x to <vscale x 8 x i16>
  %yhead = insertelement <vscale x 8 x i8> poison, i8 %y, i32 0
  %ysplat = shufflevector <vscale x 8 x i8> %yhead, <vscale x 8 x i8> poison, <vscale x 8 x i32> zeroinitializer
  %yzv = zext <vscale x 8 x i8> %ysplat to <vscale x 8 x i16>
  ...
}
```

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