[CostModel][SVE] Correct cost model of SK_Splice shuffles for <vscale x 1 x Ty> vector types.

AArch64TTIImpl::getSpliceCost() is now used more aggressively and
LNT (MultiSource/Benchmarks/mafft) exposed a failure case for
<vscale x 1 x i1>. I've tested other element types and whilst they
can be costed they cannot be code generated, so this patch returns
InstructionCost::getInvalid() for all cases.