Use bforest::Map for representing live ranges.

Get rid of the per-value Vec in the LiveRange data type and use a
bforest::Map instead to represent the live-in intervals for non-local
live ranges.

This has some advantages:

- The memory footprint of a local live range is reduced from 40 to 20
  bytes, and
- Clearing the Liveness data structure is now a constant time operation
  which doesn't call free().
- The potentially quadratic behavior when computing large live ranges is
  controlled by the logarithmic B-tree operations.

lib/cretonne/src/regalloc/live_value_tracker.rs

@@ -189,6 +189,7 @@ impl LiveValueTracker {
             let idom_live_list = self.idom_sets.get(&idom).expect(
                 "No stored live set for dominator",
             );
+            let ctx = liveness.context(layout);
             // Get just the values that are live-in to `ebb`.
             for &value in idom_live_list.as_slice(&self.idom_pool) {
                 let lr = liveness.get(value).expect(
@@ -196,7 +197,7 @@ impl LiveValueTracker {
                 );
 
                 // Check if this value is live-in here.
-                if let Some(endpoint) = lr.livein_local_end(ebb, layout) {
+                if let Some(endpoint) = lr.livein_local_end(ebb, ctx) {
                     self.live.push(value, endpoint, lr);
                 }
             }