[DAG] isSplatValue - don't attempt to merge any BITCAST sub elements if they contain UNDEFs

We still haven't found a solution that correctly handles 'don't care' sub elements properly - given how close it is to the next release branch, I'm making this fail safe change and we can revisit this later if we can't find alternatives.

NOTE: This isn't a reversion of D128570 - it's the removal of undef handling across bitcasts entirely

Fixes #56520