move some things around, reorder some instructions

constant 0 is used several times. load it once per call in horiz,
once per loop in vert.

separate saturating instructions to avoid stalls.

just use one usub8 call to set GE flags, rather than uqsub8 followed by
usub8 w/ 0

document some stalls for further consideration

Change-Id: Ic3877e0ddbe314bb8a17fd5db73501a7d64570ec