fortran: Update index extraction code. [PR102043]

fortran: Update index extraction code. [PR102043]

This avoids a regression on hollerith4.f90 and hollerith6.f90 later in
the patch series when code generation for array references is changed
to use pointer arithmetic.

The problem comes from the extraction of the array index from an
ARRAY_REF tree, which doesn’t work if the tree is not an ARRAY_REF
any more.

This updates the code generated for remaining size evaluation to work
with a source tree that uses either array indexing or pointer
arithmetic.

	PR fortran/102043

gcc/fortran/ChangeLog:

	* trans-io.cc: Add handling for the case where the array
	is referenced using pointer arithmetic.