Prepare for inlining of SUM intrinsic

Find calls to FortranASum{Real8,Integer4}, check for dim and mask
arguments being absent - then produce an inlineable simple
version of the sum function.

(No longer a prototype, please review for push to llvm/main - not sure how to make Phabricator update the review with actual commit message)

Reviewed By: peixin, awarzynski

Differential Revision: https://reviews.llvm.org/D125407