This changes the expanded code generated for if-expressions of 1-dimensional
arrays to create a static temporary on the stack if a small upper bound can
be computed for the length of a subtype covering the result.  Static stack
allocation is preferred over dynamic allocation for code generation purpose.

This also contains a couple of enhancements to the support code for checks,
so as to avoid generating useless checks during the modified expansion.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* checks.adb (Apply_Length_Check_On_Assignment): Return early if
	the Suppress_Assignment_Checks flag is set.
	(Selected_Range_Checks): Deal with conditional expressions.
	* exp_ch4.adb (Too_Large_Length_For_Array): New constant.
	(Expand_Concatenate): Use it in lieu of Too_Large_Max_Length.
	(Expand_N_If_Expression): If the result has a unidimensional array
	type but the dependent expressions have constrained subtypes with
	known bounds, create a static temporary on the stack with a subtype
	covering the result.
	(Get_First_Index_Bounds): Deal with string literals.
	* uintp.ads (Uint_256): New deferred constant.
	* sinfo.ads (Suppress_Assignment_Checks): Document new usage.