Boost the power of phi node constant folding slightly: if all

operands are the phi node itself or undef, then return undef.
This logic already existed at a higher level so in practice it
shouldn't make the slightest difference.  Note that this code
could be replaced by a call to PN->hasConstantValue().  However
since we bail out the moment we see a non-constant operand, it
is more efficient to have a specialized version of that logic.

llvm-svn: 119041