Replace assert! with debug_assert! in production code paths.

This allows the assertions to be disabled in release builds, so that
the code is faster and smaller, at the expense of not performing the
checks. Assertions can be re-enabled in release builds with the
debug-assertions flag in Cargo.toml, as the top-level Cargo.toml
file does.

lib/cretonne/src/regalloc/coalescing.rs

@@ -238,7 +238,7 @@ impl<'a> Context<'a> {
                 // 1. It is defined in a dominating EBB and live-in to `ebb`.
                 // 2. If is itself a parameter value for `ebb`. This case should already have been
                 //    eliminated by `isolate_conflicting_params()`.
-                assert!(
+                debug_assert!(
                     lr.def() != ebb.into(),
                     "{} parameter {} was missed by isolate_conflicting_params()",
                     ebb,
@@ -494,8 +494,8 @@ impl<'a> Context<'a> {
         // Second everything else in reverse layout order. Again, short forward branches get merged
         // first. There can also be backwards branches mixed in here, though, as long as they are
         // not loop backedges.
-        assert!(self.predecessors.is_empty());
-        assert!(self.backedges.is_empty());
+        debug_assert!(self.predecessors.is_empty());
+        debug_assert!(self.backedges.is_empty());
         for (pred_ebb, pred_inst) in self.cfg.pred_iter(ebb) {
             if self.preorder.dominates(ebb, pred_ebb) {
                 self.backedges.push(pred_inst);
@@ -957,7 +957,8 @@ impl VirtualCopies {
 
     /// Indicate that `param` is now fully merged.
     pub fn merged_param(&mut self, param: Value, func: &Function) {
-        assert_eq!(self.params.pop(), Some(param));
+        let popped = self.params.pop();
+        debug_assert_eq!(popped, Some(param));
 
         // The domtree pre-order in `self.params` guarantees that all parameters defined at the
         // same EBB will be adjacent. This means we can see when all parameters at an EBB have been