Ensure the first predecessor of a loop is the pre header.

Note that the check in ssa_phi_elimination.cc was very defensive:
it does not affect the outcome of the algorithm whether the
loop phi takes itself as the first input.

It makes things consistent to always have the pre header as first
input.

Change-Id: Ic86248c1f38af67f7432782f6deefae1f4bf1ab6