Add -ddump-* flags for Core optimisations (#23491)

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a testcase to the testsuite.

This adds flags and corresponding user guide documentation for dumping some additional core passes (#23491 (closed)).