[Ada] Cleanup detection of No_Elist with No and Present

[Ada] Cleanup detection of No_Elist with No and Present

Replace equality and inequality operators with calls to No and Present.
Offending occurrences found with:

$ grep -n " /\?= No_Elist" *.adb

Code cleanup only; semantics is unaffected.

gcc/ada/

	* exp_ch11.adb, exp_ch5.adb, exp_prag.adb, gnat_cuda.adb,
	sem_ch12.adb, sem_ch3.adb, sem_ch6.adb, sem_util.adb,
	treepr.adb: Replace /= and = operators with No and Present,
	respectively.