Individual compilation passes call the corresponding timing::*()
function and hold on to their timing token while they run. This causes
nested per-pass timing information to be recorded in thread-local
storage.
The --time-passes command line option prints a pass timing report to
stdout.
Define two public iterator types in the flowgraph module, PredIter and
SuccIter, which are by-value iterators over an EBB's predecessors and
successors respectively.
Provide matching pred_iter() and succ_iter() methods for inspecting the
CFG. Remove the get_predecessors() method which returned a slice.
Update the uses of get_predecessors(), none of which depended on it
being a slice.
This abstraction makes it possible to change the internal representation
of the CFG.
