[SDAG] Treat DemandedElts argument to isSplatVector as splat for scalable vectors...

[SDAG] Treat DemandedElts argument to isSplatVector as splat for scalable vectors [nfc]

The previous code used a APInt(1, 0) to represent the demanded elts of a scalable vector, and then ignored that argument if type was scalable.  This was inconsistent with the UndefElts parameter which is set to either APInt(1, 0) or APInt(1,1) - that is, implicitly broadcast across all lanes.  Particularly since the undef code relied on the DemandedElts parameter having bitwidth 1 to achieve that result!

This change switches the demanded parameter to APInt(1,1), documents the broadcast semantics, and takes advantage of it to remove one special case for scalable vectors which is no longer required.