Enable support for precise scheduling of the instruction selection

DAG. Disable using "-disable-sched-cycles".

For ARM, this enables a framework for modeling the cpu pipeline and
counting stalls. It also activates several heuristics to drive
scheduling based on the model. Scheduling is inherently imprecise at
this stage, and until spilling is improved it may defeat attempts to
schedule. However, this framework provides greater control over
tuning codegen.

Although the flag is not target-specific, it should have very little
affect on the default scheduler used by x86. The only two changes that
affect x86 are:
- scheduling a high-latency operation bumps the current cycle so independent
  operations can have their latency covered. i.e. two independent 4
  cycle operations can produce results in 4 cycles, not 8 cycles.
- Two operations with equal register pressure impact and no
  latency-based stalls on their uses will be prioritized by depth before height
  (height is irrelevant if no stalls occur in the schedule below this point).

llvm-svn: 123971

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