ART: Fix DeadPhiHandling creating >2 equivalents

Run test 531 failed a DCHECK because the DeadPhiHandling algorithm
would generate three phis of the same type:
  (0) loop phi [#0, ref] with equivalents void & object
  (1) type object equivalent by its first input => int & int
  (2) request object equivalent by a user => int & int & object
  (3) type second => int & object & object (dead)
  (4) type first => object & object & object

This patch fixes the issue by skipping (1) when the phi already has
a type, thus not creating the third equivalent for the phi's user.

Change-Id: I00c990a5982ddc1f7de013f72bbcfb1c649a6e5f