[Ada] Reuse Is_Rewrite_Substitution where possible

[Pierre-Marie de Rodat]

Use a high-level Is_Rewrite_Substitution instead of a low-level inequality,
with the intention to improve the code easier to read. Semantics unaffected,
so no test provided.

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

2018-05-30  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* errout.adb, exp_aggr.adb, exp_ch7.adb, exp_util.adb, lib.adb,
	sem_ch13.adb, sem_ch4.adb, sem_res.adb, sem_util.adb
	(Has_Original_Node): Refactor to use Is_Rewrite_Substitution.
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