DAG: Add helper for testing constant values

There are helpers for testing for constant or constant build_vector,
and for splat ConstantFP vectors, but not for a constantfp or
non-splat ConstantFP vector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290317 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed