[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!

llvm-svn: 333601