[Ada] Remove redundant predicate checks

[Ada] Remove redundant predicate checks

This patch removes redundant dynamic predicate checks generated by type
conversions in various contexts. The patch also recognizes additional
such checks that can be evaluated statically when applied to constant
values.

No simple test available.

2019-07-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_N_Type_Conversion): If a predicate check
	is generated, analyze it with range check suppressed, because
	that check has been previously applied.
	* exp_ch5.adb (Expand_N_Assignment_Statement): If the RHS is a
	type conversion to the type of the LHS, do not apply a predicate
	check to the RHS because it will have been generated already
	during its expansion.
	* exp_ch6.adb (Can_Fold_Predicate_Call): Extend processing to
	handle a predicate check on a constant entity whose value is
	static.

From-SVN: r273395