[IR] Remove override of operator new from PHINode.

This just forwarded to the same signature in User. The version in User is protected so there's no danger of anyone outside of PHINode constructing with the wrong operator new. All PHINodes are created by a static Create function in PHINode.

I believe at one point in history this called User::operator new(s, 0) so it was useful then.

llvm-svn: 305255