[NewPM] Add OptimizerEarly module extension point

VectorizerStart extension is module callback in old PM, but is function
callback in new PM. We lack a module extension point between end of
buildModuleSimplificationPipeline and the function optimization
(including vectorizer) pipeline. So this patch adds a new module
extension point before the function optimization pipeline.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D122296