[VP] Add method for looking up functional intrinsic ID for VP. NFC (#66190)

This adds a helper method to get the ID of the functionally equivalent
intrinsic, similar to the existing getFunctionalOpcodeForVP and
getConstrainedIntrinsicIDForVP methods.

Not sure if it's notable or not, but I can't find any existing uses of
VP_PROPERTY_FUNCTIONAL_INTRINSIC?

It could potentially be used in #65706 to scalarize VP intrinsics.