[Flang] Move several definitions to IntrinsicCall header for code cleanliness and reusability

In the future we intend to add support for many PowerPC-specific intrinsics that ideally will exist in a separate new PPCIntrinsicCall file. But first we need to move definitions to the IntrinsicCall header file to increase code cleanliness and readability and to make code reusable for when we add PPCIntrinsicCall.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D151715