[NFC] Expose the -Oz module optimization pipeline to opt

This exposes the module optimization pipeline as a pass that can be
applied stand-alone when using 'opt'. This helps ml inliner training
scenarios, where we start with IR captured right before inlining,
perform the inlining (-scc-oz-module-inliner) and then want to continue
and observe the final IR (where this patch comes into play). We can then
apply llc on the resulting IR to continue compilation down to native.

Differential Revision: https://reviews.llvm.org/D86224