useless-if-before-free: freed expr may have white-space differences

useless-if-before-free: freed expr may have white-space differences

* build-aux/useless-if-before-free: Recognize cases in which the
freed expression differs from the tested one in embedded white
space, e.g., if (p[i + 1]) free(p[i+1]).  Correct thinko in prev:
$1 was used, so we can't make any regexp shy.  Improved tests now
detect this.