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/dominator_tree.rs

@@ -197,7 +197,7 @@ impl DominatorTree {
             }
         }
 
-        assert_eq!(a.0, b.0, "Unreachable block passed to common_dominator?");
+        debug_assert_eq!(a.0, b.0, "Unreachable block passed to common_dominator?");
 
         // We're in the same EBB. The common dominator is the earlier instruction.
         if layout.cmp(a.1, b.1) == Ordering::Less {
@@ -241,7 +241,7 @@ impl DominatorTree {
     pub fn clear(&mut self) {
         self.nodes.clear();
         self.postorder.clear();
-        assert!(self.stack.is_empty());
+        debug_assert!(self.stack.is_empty());
         self.valid = false;
     }
 
@@ -539,7 +539,7 @@ impl DominatorTreePreorder {
     /// Recompute this data structure to match `domtree`.
     pub fn compute(&mut self, domtree: &DominatorTree, layout: &Layout) {
         self.nodes.clear();
-        assert_eq!(self.stack.len(), 0);
+        debug_assert_eq!(self.stack.len(), 0);
 
         // Step 1: Populate the child and sibling links.
         //
@@ -557,7 +557,7 @@ impl DominatorTreePreorder {
         }
 
         // Step 2. Assign pre-order numbers from a DFS of the dominator tree.
-        assert!(self.stack.len() <= 1);
+        debug_assert!(self.stack.len() <= 1);
         let mut n = 0;
         while let Some(ebb) = self.stack.pop() {
             n += 1;