Refactor array descriptor field access

Refactor array descriptor field access

This refactors the array descriptor component access tree building
to commonize code into new helpers to provide a single place to
fix correctness issues with respect to TBAA.

The only interesting part is the gfc_conv_descriptor_data_get change
to drop broken special-casing of REFERENCE_TYPE desc which, when hit,
would build invalid GENERIC trees, missing an INDIRECT_REF before
subsetting the descriptor with a COMPONENT_REF.

2020-10-16  Richard Biener  <rguenther@suse.de>

gcc/fortran/ChangeLog:
	* trans-array.c (gfc_get_descriptor_field): New helper.
	(gfc_conv_descriptor_data_get): Use it - drop strange
	REFERENCE_TYPE handling and make sure we don't trigger it.
	(gfc_conv_descriptor_data_addr): Use gfc_get_descriptor_field.
	(gfc_conv_descriptor_data_set): Likewise.
	(gfc_conv_descriptor_offset): Likewise.
	(gfc_conv_descriptor_dtype): Likewise.
	(gfc_conv_descriptor_span): Likewise.
	(gfc_get_descriptor_dimension): Likewise.
	(gfc_conv_descriptor_token): Likewise.
	(gfc_conv_descriptor_subfield): New helper.
	(gfc_conv_descriptor_stride): Use it.
	(gfc_conv_descriptor_lbound): Likewise.
	(gfc_conv_descriptor_ubound): Likewise.