Add a liveness verifier.

The liveness verifier will check that the live ranges are consistent
with the function. It runs as part of the register allocation pipeline
when enable_verifier is set.

The initial implementation checks the live ranges, but not the
ISA-specific constraints and affinities.

lib/cretonne/src/context.rs

@@ -81,7 +81,7 @@ impl Context {
     /// Run the register allocator.
     pub fn regalloc(&mut self, isa: &TargetIsa) -> CtonResult {
         self.regalloc
-            .run(isa, &mut self.func, &self.cfg, &self.domtree);
+            .run(isa, &mut self.func, &self.cfg, &self.domtree)?;
         self.verify_if(isa)
     }
 }