freedreno/ir3: cleanup double cmps.s from frontend

freedreno/ir3: cleanup double cmps.s from frontend

Since we cannot mov into a predicate register, the frontend uses a
'cmps.s p0.x, cond, 0' as a stand-in for mov to p0.x.  It does this
since it has no way to know that the source cond instruction (ie.
for a kill, br, etc) will only be used to write the predicate reg.
Detect this, and re-write the instruction writing p0.x to skip the
original cmps.[sfu].  (It is done like this, rather than re-writing
the dest of the first cmps.[sfu] in case the first cmps.[sfu]
actually has other users.)

Signed-off-by: Rob Clark <robclark@freedesktop.org>