[MCA] Moved the logic that updates register dependencies from DispatchStage to Regist...

[MCA] Moved the logic that updates register dependencies from DispatchStage to RegisterFile. NFC

DispatchStage should always delegate to an object of class RegisterFile the task
of updating data dependencies.  ReadState and WriteState objects should not be
modified directly by DispatchStage.
This patch also renames stage IS_AVAILABLE to IS_DISPATCHED.

llvm-svn: 353170