This appears to be the last missing piece for the masked merge pattern handling in the backend.
This is PR37104.
PR6773 will introduce an IR canonicalization that is likely bad for the end assembly.
Previously, andps+andnps / bsl would be generated. (see @out)
Now, they would no longer be generated (see @in), and we need to make sure that they are generated.
Before reviewing this, it is best to finish with the scalar part.