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/abi.rs

@@ -150,7 +150,7 @@ pub fn legalize_abi_value(have: Type, arg: &AbiParam) -> ValueConversion {
     match have_bits.cmp(&arg_bits) {
         // We have fewer bits than the ABI argument.
         Ordering::Less => {
-            assert!(
+            debug_assert!(
                 have.is_int() && arg.value_type.is_int(),
                 "Can only extend integer values"
             );
@@ -163,8 +163,8 @@ pub fn legalize_abi_value(have: Type, arg: &AbiParam) -> ValueConversion {
         // We have the same number of bits as the argument.
         Ordering::Equal => {
             // This must be an integer vector that is split and then extended.
-            assert!(arg.value_type.is_int());
-            assert!(have.is_vector());
+            debug_assert!(arg.value_type.is_int());
+            debug_assert!(have.is_vector());
             ValueConversion::VectorSplit
         }
         // We have more bits than the argument.