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/legalizer/split.rs

@@ -127,7 +127,7 @@ fn split_any(
     while let Some(repair) = repairs.pop() {
         for (_, inst) in cfg.pred_iter(repair.ebb) {
             let branch_opc = pos.func.dfg[inst].opcode();
-            assert!(
+            debug_assert!(
                 branch_opc.is_branch(),
                 "Predecessor not a branch: {}",
                 pos.func.dfg.display_inst(inst, None)
@@ -198,7 +198,7 @@ fn split_value(
             // This is an instruction result. See if the value was created by a `concat`
             // instruction.
             if let InstructionData::Binary { opcode, args, .. } = pos.func.dfg[inst] {
-                assert_eq!(num, 0);
+                debug_assert_eq!(num, 0);
                 if opcode == concat {
                     reuse = Some((args[0], args[1]));
                 }