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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

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

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