[MCA][Scheduler] Use latency information to further classify busy instructions.

This patch introduces a new instruction stage named 'IS_PENDING'.
An instruction transitions from the IS_DISPATCHED to the IS_PENDING stage if
input registers are not available, but their latency is known.

This patch also adds a new set of instructions named 'PendingSet' to class
Scheduler. The idea is that the PendingSet will only contain instructions that
have reached the IS_PENDING stage.
By construction, an instruction in the PendingSet is only dependent on
instructions that have already reached the execution stage. The plan is to use
this knowledge to identify bottlenecks caused by data dependencies (see
PR37494).

Differential Revision: https://reviews.llvm.org/D58066

llvm-svn: 353937