Fix a bug in the type analysis phase of optimizing.

Dex code can lead to the creation of a phi with one
float input and one integer input. Since the SSA builder trusts
the verifier, it assumes that the integer input must be converted
to float. However, when the register is not used afterwards, the
verifier hasn't ensured that. Therefore, the compiler must remove
the phi prior to doing type propagation.

Change-Id: Idcd51c4dccce827c59d1f2b253bc1c919bc07df5