[ValueTracking] Handle two PHIs in isKnownNonEqual()

loop:
  %cmp.0 = phi i32 [ 3, %entry ], [ %inc, %loop ]
  %pos.0 = phi i32 [ 1, %entry ], [ %cmp.0, %loop ]
  ...
  %inc = add i32 %cmp.0, 1
  br label %loop

On above example, %pos.0 uses previous iteration's %cmp.0 with backedge
according to PHI's instruction's defintion. If the %inc is not same among
iterations, we can say the two PHIs are not same.

Differential Revision: https://reviews.llvm.org/D98422