[Schedule] Add a MultiHazardRecognizer

This adds a MultiHazardRecognizer and starts to make use of it in the
ARM backend. The idea of the class is to allow multiple independent
hazard recognizers to be added to a single base MultiHazardRecognizer,
allowing them to all work in parallel without requiring them to be
chained into subclasses. They can then be added or not based on cpu or
subtarget features, which will become useful in the ARM backend once
more hazard recognizers are being used for various things.

This also renames ARMHazardRecognizer to ARMHazardRecognizerFPMLx in the
process, to more clearly explain what that recognizer is designed for.

Differential Revision: https://reviews.llvm.org/D72939