[Ada] Reuse Is_Rewrite_Substitution where possible

[Ada] Reuse Is_Rewrite_Substitution where possible

Replace comparisons of Original_Node with semantically equivalent but
high-level calls to Is_Rewrite_Substitution. Offending occurrences found
with:

  $ grep -n "Original_Node (\([A-Za-z_]\+\)) /\?= \1" *.adb

Code cleanup only; semantics is unaffected.

gcc/ada/

	* atree.adb, exp_ch6.adb, exp_ch9.adb, ghost.adb, sem_ch3.adb,
	sem_ch4.adb, sem_res.adb, sem_util.adb: Use
	Is_Rewrite_Substitution where possible.