Add a compilation pass timing facility.

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.

lib/cretonne/src/dominator_tree.rs

@@ -5,6 +5,7 @@ use flowgraph::{ControlFlowGraph, BasicBlock};
 use ir::{Ebb, Inst, Function, Layout, ProgramOrder, ExpandedProgramPoint};
 use ir::instructions::BranchInfo;
 use packed_option::PackedOption;
+use timing;
 
 use std::cmp::Ordering;
 
@@ -227,6 +228,7 @@ impl DominatorTree {
 
     /// Reset and compute a CFG post-order and dominator tree.
     pub fn compute(&mut self, func: &Function, cfg: &ControlFlowGraph) {
+        let _tt = timing::domtree();
         debug_assert!(cfg.is_valid());
         self.compute_postorder(func);
         self.compute_domtree(func, cfg);