[SDAG] Actually check for a non-constant splat and clarify comments
around the handling of UNDEF lanes in boolean vector content analysis.

The code before my changes here also failed to check for non-constant
splats in a buildvector. I have no idea how to trigger this, I just
spotted by inspection when trying to understand the code. It seems
extremely unlikely to be worth the trouble to teach the only caller of
this code (DAG combining setcc patterns) how to cleverly handle undef
lanes, so I've just commented more thoroughly that we're giving up
there.