[NFC intended] Refactor SinkAndHoistLICMFlags to allow others to construct...

[NFC intended] Refactor SinkAndHoistLICMFlags to allow others to construct without exposing internals

Summary:
Refactor SinkAdHoistLICMFlags from a struct to a class with accessors and constructors to allow other
classes to construct flags with meaningful defaults while not exposing LICM internal details.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>

Reviewed By: asbirlea (Alina Sbirlea)

Differential Revision: https://reviews.llvm.org/D90482