[LoopExtractor] Convert LoopExtractor from LoopPass to ModulePass

The LoopExtractor created new functions (by definition), which violates
the restrictions of a LoopPass.
The correct implementation of this pass should be as a ModulePass.
Includes reverting rL82990 implications on the LoopExtractor.

Fixes PR3082 and PR8929.

Differential Revision: https://reviews.llvm.org/D69069