add a redundant loop condition to silence GCC warnings

See:
#94
#183
#189

c/blake3.c

@@ -345,7 +345,12 @@ INLINE void compress_subtree_to_parent_node(
   // compress_subtree_wide() returns more than 2 chaining values. Condense
   // them into 2 by forming parent nodes repeatedly.
   uint8_t out_array[MAX_SIMD_DEGREE_OR_2 * BLAKE3_OUT_LEN / 2];
-  while (num_cvs > 2) {
+  // The second half of this loop condition, num_cvs <= MAX_SIMD_DEGREE_OR_2,
+  // is always true. compress_subtree_wide() will never return more than
+  // MAX_SIMD_DEGREE_OR_2 items. But GCC isn't aware of that fact, and on
+  // platforms where MAX_SIMD_DEGREE_OR_2 == 2, it emits spurious warnings
+  // here. The second half of this loop condition silences those warnings.
+  while (num_cvs > 2 && num_cvs <= MAX_SIMD_DEGREE_OR_2) {
     num_cvs =
         compress_parents_parallel(cv_array, num_cvs, key, flags, out_array);
     memcpy(cv_array, out_array, num_cvs * BLAKE3_OUT_LEN);