[NewGVN] Fix a consistent order for phi nodes operands.

The way we currently define congruency for two PHIExpression(s) is:

1) The operands to the phi functions are congruent
2) The PHIs are defined in the same BasicBlock.

NewGVN works under the assumption that phi operands are in predecessor
order, or at least in some consistent order. OTOH, is valid IR:

patatino:
  %meh = phi i16 [ %0, %winky ], [ %conv1, %tinky ]
  %banana = phi i16 [ %0, %tinky ], [ %conv1, %winky ]
  br label %end

and the in-memory representations of the two SSA registers have an
inconsistent order. This violation of NewGVN assumptions results into
two PHIs found congruent when they're not. While we think it's useful
to have always a consistent order enforced, let's fix this in NewGVN
sorting uses in predecessor order before creating a PHI expression.

Differential Revision:  https://reviews.llvm.org/D32990

llvm-svn: 302552