[MCA] Slightly refactor class RetireControlUnit, and add the ability to override...

[MCA] Slightly refactor class RetireControlUnit, and add the ability to override the mask of used buffered resources in class mca::Instruction. NFCI

This patch teaches the RCU how to peek 'next' RCUTokens. A new method has been
added to the RetireControlUnit class with the goal of minimizing the complexity
of follow-up patches that will enable macro-fusion support in mca.

This patch also adds method Instruction::getNumMicroOpcodes() to simplify common
interactions with the instruction descriptor (a pattern quite common in some
pipeline stages).

Added the ability to override the default set of consumed scheduler resources
(this -again- is to simplify future patches that add support for macro-op fusion).

No functional change intended.

llvm-svn: 369010