Fix infinite simulation cycles in SSA propagator.

Fix infinite simulation cycles in SSA propagator.

This fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/1159.  I
had missed a nuance in the original algorithm.  When simulating Phi
instructions, the SSA edges out of a Phi instruction should never be
added to the list of edges to simulate.

Phi instructions can be in SSA def-use cycles with other Phi
instructions.  This was causing the propagator to fall into an infinite
loop when the same def-use edge kept being added to the queue.

The original algorithm in the paper specifically separates the visit of
a Phi instruction vs the visit of a regular instruction.  This fix makes
the implementation match the original algorithm.