Adapt to new LLVM pass manager

LLVM has a new pass manager which means we have to change how we pass the options of passes we want it to run.

Here is the guide about migrating to using the new pass manager: https://llvm.org/docs/NewPassManager.html#invoking-opt

To give ourselves some breathing room to migrate to the new pass manager we pass -enable-new-pm=0, this alleviates the modicum of thinking require for this migration in the 9.6 timescale.

A WIP patch is !8999 (closed)