[MCA] Refactor class LSUnit. NFCI

This should be the last bit of refactoring in preparation for a patch that would
finally fix PR37494.

This patch introduces the concept of memory dependency groups (class
MemoryGroup) and "Load/Store Unit token" (LSUToken) to track the status of a
memory operation.

A MemoryGroup is a node of a memory dependency graph. It is used internally to
classify memory operations based on the memory operations they depend on.  Let I
and J be two memory operations, we say that I and J equivalent (for the purpose
of mapping instructions to memory dependency groups) if the set of memory
operations they depend depend on is identical.

MemoryGroups are identified by so-called LSUToken (a unique group identifier
assigned by the LSUnit to every group). When an instruction I is dispatched to
the LSUnit, the LSUnit maps I to a group, and then returns a LSUToken.
LSUTokens are used by class Scheduler to track memory dependencies.

This patch simplifies the LSUnit interface and moves most of the implementation
details to its base class (LSUnitBase). There is no user visible change to the
output.

llvm-svn: 361950