Report changes in simpleCSE

[alex-mckenna]

The simpleCSE transformation previously never reported when it had
changed the resulting AST. With the fix from f68204f this meant
that CSE would often cause failures when using DebugApplied or
higher.

Fixes #1846

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files
• Make sure the testsuite can detect such errors move to Improved -fclash-debug-* flags #1847

[leonschoorl]

The testsuite should have caught this

[alex-mckenna]

The testsuite should have caught this

Discussed offline, the testsuite runs with DebugSilent which is too low to catch this failure (which needs DebugApplied or higher). The best course of action is to change when checks like this are performed in #1800 which touches that logic anyway