[flang] use greedy mlir driver for stack arrays pass

In upstream mlir, the dialect conversion infrastructure is used for
lowering from one dialect to another: the passes are of the form
XToYPass. Whereas, transformations within the same dialect tend to use
applyPatternsAndFoldGreedily.

In this case, the full complexity of applyPatternsAndFoldGreedily isn't
needed so we can get away with the simpler applyOpPatternsAndFold.

This change was suggested by @jeanPerier

Differential Revision: https://reviews.llvm.org/D150853