[llvm-mca] Introduce a pipeline Stage class and FetchStage.

[llvm-mca] Introduce a pipeline Stage class and FetchStage.

Summary:
    This is just an idea, really two ideas.  I expect some push-back,
    but I realize that posting a diff is the most comprehensive way to express
    these concepts.

    This patch introduces a Stage class which represents the
    various stages of an instruction pipeline.  As a start, I have created a simple
    FetchStage that is based on existing logic for how MCA produces
    instructions, but now encapsulated in a Stage.  The idea should become more concrete
    once we introduce additional stages.  The idea being, that when a stage completes,
    the next stage in the pipeline will be executed.  Stages are chained together
    as a singly linked list to closely model a real pipeline. For now there is only one stage,
    so the stage-to-stage flow of instructions isn't immediately obvious.

    Eventually, Stage will also handle event notifications, but that functionality
    is not complete, and not destined for this patch.  Ideally, an interested party
    can register for notifications from a particular stage.  Callbacks will be issued to
    these listeners at various points in the execution of the stage.
    For now, eventing functionality remains similar to what it has been in mca::Backend.
    We will be building-up the Stage class as we move on, such as adding debug output.

    This patch also removes the unique_ptr<Instruction> return value from
    InstrBuilder::createInstruction.  An Instruction pointer is still produced,
    but now it's up to the caller to decide how that item should be managed post-allocation
    (e.g., smart pointer).  This allows the Fetch stage to create instructions and
    manage the lifetime of those instructions as it wishes, and not have to be bound to any
    specific managed pointer type.  Other callers of createInstruction might have different
    requirements, and thus can manage the pointer to fit their needs.  Another idea would be to push the
   ownership to the RCU.

    Currently, the FetchStage will wrap the Instruction
    pointer in a shared_ptr.  This allows us to remove the Instruction container in
    Backend, which was probably going to disappear, or move, at some point anyways.
    Note that I did run these changes through valgrind, to make sure we are not leaking
    memory.  While the shared_ptr comes with some additional overhead it relieves us
    from having to manage a list of generated instructions, and/or make lookup calls
    to remove the instructions.

    I realize that both the Stage class and the Instruction pointer management
    (mentioned directly above) are separate but related ideas, and probably should
    land as separate patches; I am happy to do that if either idea is decent.
    The main reason these two ideas are together is that
    Stage::execute() can mutate an InstRef. For the fetch stage, the InstRef is populated
    as the primary action of that stage (execute()).  I didn't want to change the Stage interface
    to support the idea of generating an instruction.  Ideally, instructions are to
    be pushed through the pipeline.  I didn't want to draw too much of a
    specialization just for the fetch stage.  Excuse the word-salad.

Reviewers: andreadb, courbet, RKSimon

Reviewed By: andreadb

Subscribers: llvm-commits, mgorny, javed.absar, tschuett, gbedwell

Differential Revision: https://reviews.llvm.org/D46741

llvm-svn: 332390