[llvm] [SDAG] Implement missing legalization for `ISD::VECTOR_FIND_LAST_ACTIVE` (PR #180290)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Thanks!
> 
> I think this may also fix a crash on WebAssembly, for which we currently return invalid costs as workaround? If so, would be great if we could remove the cost workaround
> 
> > For a reproducer of the wasm crash, see
> > https://github.com/kripken/emscripten_/blob/vectorize_crash/test/vectorize_crash.cpp
> > clang++ -target wasm32-unknown-emscripten -O3 -c vectorize_crash.cpp

It does now that I've added scalarization for the `<1 x i1>` case. 

https://github.com/llvm/llvm-project/pull/180290