Decompose PhasedISwap with an exponent of 1.

This should only be called if the Gate has an exponent of 1 - otherwise, decompose_phased_iswap_into_syc_precomputed should be used instead. The advantage of using this function is that the resulting circuit will be smaller.

phase_exponent The exponent on the Z gates.
a First qubit id to operate on
b Second qubit id to operate on

a Cirq program implementing the Phased ISWAP gate