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/binemit/mod.rs

@@ -110,7 +110,7 @@ where
     let mut divert = RegDiversions::new();
     for ebb in func.layout.ebbs() {
         divert.clear();
-        assert_eq!(func.offsets[ebb], sink.offset());
+        debug_assert_eq!(func.offsets[ebb], sink.offset());
         for inst in func.layout.ebb_insts(ebb) {
             emit_inst(func, inst, &mut divert, sink);
         }

lib/cretonne/src/binemit/relaxation.rs

@@ -60,7 +60,7 @@ pub fn relax_branches(func: &mut Function, isa: &TargetIsa) -> Result<CodeOffset
         while let Some(ebb) = cur.next_ebb() {
             // Record the offset for `ebb` and make sure we iterate until offsets are stable.
             if cur.func.offsets[ebb] != offset {
-                assert!(
+                debug_assert!(
                     cur.func.offsets[ebb] < offset,
                     "Code shrinking during relaxation"
                 );
@@ -111,7 +111,7 @@ fn fallthroughs(func: &mut Function) {
                 Opcode::Fallthrough => {
                     // Somebody used a fall-through instruction before the branch relaxation pass.
                     // Make sure it is correct, i.e. the destination is the layout successor.
-                    assert_eq!(destination, succ, "Illegal fall-through in {}", ebb)
+                    debug_assert_eq!(destination, succ, "Illegal fall-through in {}", ebb)
                 }
                 Opcode::Jump => {
                     // If this is a jump to the successor EBB, change it to a fall-through.