[Ada] Deal with second specific superflat case in Optimize_Length_Comparison

[Ada] Deal with second specific superflat case in Optimize_Length_Comparison

2020-06-17  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch4.adb (Optimize_Length_Comparison): New local variable to
	record whether this may be a dynamic superflat case.
	(Is_Optimizable): Accept 0 as lower bound and set it in this case,
	but return false if the operand is not a length too.
	(Rewrite_For_Equal_Lengths): New procedure.
	Optimize the comparison of two lengths in the superflat case when
	the arrays have the same bounds.