[Ada] Add support for compile time evaluation of Shift_Right_Arithmetic

[Pierre-Marie de Rodat]

After recent changes to evaluate logical Shift operations at compile
time, do the same for the more complicated case of
Shift_Right_Arithmetic as well as for Shift_Right/Shift_Left for signed
integers.

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

gcc/ada/

	* sem_eval.adb (Eval_Intrinsic_Call, Fold_Shift): Add support
	for Shift_Right_Arithmetic and for signed integers.
	* exp_ch4.adb (Expand_N_Op_Rotate_Left,
	Expand_N_Op_Rotate_Right, Expand_N_Op_Shift_Left,
	Expand_N_Op_Shift_Right_Arithmetic): Minor reformatting and code
	cleanup to ensure a consistent handling. Update comments and add
	assertion.
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