[Flang,MLIR,OpenMP] Fix a few tests that were not converting to LLVM

[Flang,MLIR,OpenMP] Fix a few tests that were not converting to LLVM

A few OpenMP tests were retaining the FIR operands even after running
the LLVM conversion pass. To fix these tests the legality checkes for
OpenMP conversion are made stricter to include operands and results.
The Flush, Single and Sections operations are added to conversions or
legality checks. The RegionLessOpConversion is appropriately renamed
to clarify that it works only for operations with Variable operands.
The operands of the flush operation are changed to match those of
Variable Operands.

Fix for an OpenMP issue mentioned in
https://github.com/llvm/llvm-project/issues/55210.

Reviewed By: shraiysh, peixin, awarzynski

Differential Revision: https://reviews.llvm.org/D127092