Do not check for a misaligned pointer twice in GC_is_visible

(refactoring)

* ptr_chck.c [!THREADS && ALIGNMENT==CPP_PTRSZ/8] (GC_is_visible): Do
not evaluate `ADDR(p)&(sizeof(ptr_t)-1)` (because `sizeof(ptr_t)` is
equal to `ALIGNMENT` and `ADDR(p)&(ALIGNMENT-1)` is zero).

ptr_chck.c

@@ -198,9 +198,12 @@ GC_is_visible(void *p)
         goto fail;
       break;
     case GC_DS_BITMAP:
-      if ((ptr_t)p - base >= (ptrdiff_t)PTRS_TO_BYTES(BITMAP_BITS)
-          || (ADDR(p) & (sizeof(ptr_t) - 1)) != 0)
+      if ((ptr_t)p - base >= (ptrdiff_t)PTRS_TO_BYTES(BITMAP_BITS))
         goto fail;
+#  if ALIGNMENT != CPP_PTRSZ / 8
+      if ((ADDR(p) & (sizeof(ptr_t) - 1)) != 0)
+        goto fail;
+#  endif
       if (!(((word)1 << (CPP_WORDSZ - 1 - (word)((ptr_t)p - base))) & descr))
         goto fail;
       break;