[flang] Fix compilation after rename of Loop dialect to scf

Summary: Flang was not compiling correctly after the renaming of Loop dialect to SCF. This patch fixes the problem.

Reviewers: ftynse, DavidTruby

Reviewed By: ftynse

Subscribers: aartbik, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79723