[Ada] Small cleanup in constraint checking code for allocators

[Ada] Small cleanup in constraint checking code for allocators

2020-06-18  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_Allocator_Expression): Apply constraint
	and predicate checks for the qualified expression on entry,
	followed by constraint and predicate checks for the allocator
	itself, and return early if this results in a static error.
	(Expand_N_Allocator): Do not do the same here.  Instead apply
	constraint and predicate checks for arrays in the subtype
	indication case.
	* exp_ch5.adb (Expand_N_Assignment_Statement): Do not apply
	range checks to allocators here.