[llvm] [RISCV] Optimize divide by constant for VP intrinsics (PR #125991)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Was there are particular reason for doing this in DAGCombiner as opposed to InstCombine? E.g. I think instcombine currently handles the udiv of max case:

```llvm

define <vscale x 2 x i64> @f(<vscale x 2 x i64> %v) {
  %x = udiv <vscale x 2 x i64> %v, splat (i64 18446744073709551615)
  ret <vscale x 2 x i64> %x
}

->

define <vscale x 2 x i64> @f(<vscale x 2 x i64> %v) {
  %1 = icmp eq <vscale x 2 x i64> %v, splat (i64 -1)
  %x = zext <vscale x 2 x i1> %1 to <vscale x 2 x i64>
  ret <vscale x 2 x i64> %x
}
```

https://github.com/llvm/llvm-project/pull/125991