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/unreachable_code.rs

@@ -4,6 +4,7 @@ use cursor::{Cursor, FuncCursor};
 use dominator_tree::DominatorTree;
 use flowgraph::ControlFlowGraph;
 use ir;
+use timing;
 
 /// Eliminate unreachable code.
 ///
@@ -16,6 +17,7 @@ pub fn eliminate_unreachable_code(
     cfg: &mut ControlFlowGraph,
     domtree: &DominatorTree,
 ) {
+    let _tt = timing::unreachable_code();
     let mut pos = FuncCursor::new(func);
     while let Some(ebb) = pos.next_ebb() {
         if domtree.is_reachable(ebb) {