[NewGVN] Fix set comparison; reflow comment
Looks like we intended to compare this->Members with Other->Members
here, but ended up comparing this->Members with this->Members. Oops. :)
Since CongruenceClass::Members is a SmallPtrSet anyway, we can probably
skip building std::sets if we're willing to write a bit more code.
This appears to be no functional change (for sufficiently lax values of
"no"): this equality check was only being called inside of an assert.
So, worst case, we'll catch more bugs in the form of assertion failures.
Thanks to d0k for noting this!