[SelectionDAG] Support scalable-vector splats in more cases

This patch adds support for scalable-vector splats in DAGCombiner's
`isConstantOrConstantVector` and `ISD::matchUnaryPredicate` functions,
which enable the SelectionDAG div/rem-by-constant optimizations for
scalable vector types.

It also fixes up one case where the UDIV optimization was generating a
SETCC without first consulting the target for its preferred SETCC result
type.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94501