[llvm-mca] Make the LSUnit a HardwareUnit, and allow derived classes to...

[llvm-mca] Make the LSUnit a HardwareUnit, and allow derived classes to implement a different memory consistency model.

The LSUnit is now a HardwareUnit, and it is owned by the mca::Context.
Derived classes can now implement a different consistency model by overriding
method `LSUnit::isReady()`.

This patch also slightly refactors the Scheduler interface in the attempt to
simplifying the interaction between ExecuteStage and the underlying Scheduler.

llvm-svn: 340176