[Ada] Simplify membership tests with N_Subprogram_Call subtype

[Pierre-Marie de Rodat]

Simplify membership test on N_Procedure_Call_Statement and
N_Function_Call using the collective N_Subprogram_Call subtype. Cleanup
only; semantics is unaffected.

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

gcc/ada/

	* exp_ch6.adb, exp_util.adb, sem_ch4.adb, sem_disp.adb,
	sem_elab.adb: Simplify membership test.
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