[SelDag] Add isIntOrFPConstant helper function.

This patch adds a new isIntOrFPConstant  helper function to check if a
SDValue is a integer of FP constant. This pattern is used in various
places.

There also are places that incorrectly just check for integer constants,
e.g. D99384, so hopefully this helper will help people avoid that issue.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D99428