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/meta/gen_instr.py

@@ -211,7 +211,7 @@ def gen_instruction_data_impl(fmt):
                     if f.has_value_list:
                         fmt.line(n + ' { ref mut args, .. } => args,')
                 fmt.line('_ => panic!("No value list: {:?}", self),')
-            fmt.line('assert!(args.is_empty(), "Value list already in use");')
+            fmt.line('debug_assert!(args.is_empty(), "Value list already in use");')
             fmt.line('*args = vlist;')