[flang] Control flow graph issues

Address several issues involving control flow graph generation and
structured code ops.

 - Fix a problem with constructs nested inside unstructured selection
   constructs. This is a general problem involving branches that are
   implied rather than explicit. It is addressed in the generic genFIR
   "wrapper" function that calls individual statement-specific genFIR calls.

 - The previous fix requires some compensating changes in IF and DO
   construct code lowering.

 - Streamline the code to generate explicit DO loop variable updates.

 - Fix a problem with the individual detailed genFIR calls made in the
   genFIR(SelectTypeConstruct) call.

 - Modify control flow graph generation to support the insertion of
   deallocation and finalization code when lowering most END <construct>
   statements.