[DAG] make isConstOrConstSplat and isConstOrConstSplatFP more accessible; NFC

As noted in:
https://reviews.llvm.org/D25685

This is the next-to-smallest step needed to enable the ComputeNumSignBits fix in that patch. 
In a minor attempt to keep some structure, we're pulling the FP helper over along with its
integer sibling, but clearly we can and should do more refactoring of the similar helper
functions in DAGCombiner and SelectionDAG to simplify and not duplicate functionality.

llvm-svn: 284421