[llvm] [WASM] Fold bitselect with splat zero (PR #147305)

[Luke Lau via llvm-commits]

lukel97 wrote:

> please can you rebase after #147472 - I think what you're after is just the undef element handling that isConstantSplatVector gives you by default

I think it's the fact that build_vectors seem to be canonicalzied to splat_vectors, and combineVSelectWithAllOnesOrZeroes only seems to pick up build_vectors because it calls these:

```c++
bool ISD::isBuildVectorAllOnes(const SDNode *N) {
  return isConstantSplatVectorAllOnes(N, /*BuildVectorOnly*/ true);
}

bool ISD::isBuildVectorAllZeros(const SDNode *N) {
  return isConstantSplatVectorAllZeros(N, /*BuildVectorOnly*/ true);
}
```

> Also, I think wasm is missing an hasAndNot override

Yes, that's exactly what we're looking for the second part of this PR on lines R13161. @badumbatish although wasm doesn't seem to have an andnot instruction, it might be worthwhile exploring if returning true is profitable for vectors anyway.

https://github.com/llvm/llvm-project/pull/147305