Avoid interference on CFG edges.

Track allocatable registers both locally and globally: Add a second
AllocatableSet which tracks registers allocated to global values without
accounting for register diversions. Since diversions are only local to
an EBB, global values must be assigned un-diverted locations that don't
interfere.

Handle the third "global" interference domain in the constraint solver in
addition to the existing "input" and "output" domains.

Extend the solver error code to indicate when a global define just can't
be allocated because there are not enough available global registers.
Resolve this problem by replacing the instruction's global defines with
local defines that are copied into their global destinations
afterwards.

lib/cretonne/src/regalloc/live_value_tracker.rs

@@ -147,6 +147,13 @@ impl LiveValueTracker {
         &self.live.values
     }
 
+    /// Get a mutable set of currently live values.
+    ///
+    /// Use with care and don't move entries around.
+    pub fn live_mut(&mut self) -> &mut [LiveValue] {
+        &mut self.live.values
+    }
+
     /// Move the current position to the top of `ebb`.
     ///
     /// This depends on the stored live value set at `ebb`'s immediate dominator, so that must have