[flang] Fix bug detecting simply contiguous component

We were always return false when testing a component for simple
contiguity. Change to check that the component is an array that is
simply continguous. Also treat a scalar component of scalar as simply
contiguous.

A pointer with bounds remapping to a complex part is a similar case
so add a test for that too.

Original-commit: flang-compiler/f18@27d76da2a44614b2c4cf4d576410372cabf66577
Reviewed-on: https://github.com/flang-compiler/f18/pull/952