Fix performance regression caused by xassert() macro

Before this commit, the xassert() macro still expanded to e.g.:

    if (unlikely(!(cond)))
        __builtin_unreachable();

...even when NDEBUG was set.

If `cond` happened to contain a function call that the compiler couldn't
prove to be side-effect free, this call would still have to be emitted
in the generated code, even when everything else could be optimized out.

This commit changes xassert() to be more in-keeping with the standard
assert() macro when NDEBUG is set; specifically, it causes the entire
statement to expand to nothing, which thus guarantees that any function
calls in the condition will be dropped, regardless of whether or not
they have side-effects.

debug.h

@@ -2,12 +2,11 @@
 
 #include "macros.h"
 
-#ifdef NDEBUG
-    #define BUG(...) UNREACHABLE()
-#else
 #define BUG(...) bug(__FILE__, __LINE__, __func__, __VA_ARGS__)
-#endif
 
+#ifdef NDEBUG
+    #define xassert(x)
+#else
     #define xassert(x) do { \
         IGNORE_WARNING("-Wtautological-compare") \
         if (unlikely(!(x))) { \
@@ -15,6 +14,7 @@
         } \
         UNIGNORE_WARNINGS \
     } while (0)
+#endif
 
 #ifndef static_assert
     #if __STDC_VERSION__ >= 201112L