fortran: Factor array descriptor references

fortran: Factor array descriptor references

Save subexpressions of array descriptor references to variables, so that
all the expressions using the descriptor as base object benefit from a
simplified reference using the variables.

This limits the size of the expressions generated in the original tree
dump, easing analysis of the code involving those expressions.
This is especially helpful with chains of array references where each
array in the chain uses a descriptor.

After optimizations, the effect of the change shouldn't be visible in
the vast majority of cases.  In rare cases it seems to permit a couple
more jump threadings.

gcc/fortran/ChangeLog:

	* trans-array.cc (gfc_conv_ss_descriptor): Move the descriptor
	expression initialisation...
	(set_factored_descriptor_value): ... to this new function.
	Before initialisation, walk the reference expression passed as
	argument and save some of its subexpressions to a variable.
	(substitute_t): New struct.
	(maybe_substitute_expr): New function.
	(substitute_subexpr_in_expr): New function.