Expose isXxxConstant() functions from SelectionDAGNodes.h (NFC)

Expose isXxxConstant() functions from SelectionDAGNodes.h (NFC)

Summary:
Many target lowerings copy-paste the code to test SDValues for known constants.
This code can instead be shared in SelectionDAG.cpp, and reused in the targets.

Reviewers: MatzeB, andreadb, tstellarAMD

Subscribers: arsenm, jyknight, llvm-commits

Differential Revision: http://reviews.llvm.org/D14945

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254085 91177308-0d34-0410-b5e6-96231b3b80d8