[flang][NFC] Remove obsolete DoLoopHelper

During the upstreaming process from fir-dev some
new builder have been introduced in the `flang/Optimizer/Builder`
directory. This patch removes the obsolete DoLoopHelper still present
in the lowering directories and makes use of the new one where needed.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D118442