freedreno/ir3: use peephole select pass

freedreno/ir3: use peephole select pass

Agressively lowering all if/else to selects in some extreme cases
results in much higher register pressure.  Using peephole select instead
with a modest threshold speeds up alu2 4x!

16 seems like a good limit, low enough to help alu2 but not too low that
it penalizes everything else.  With a bit better scheduling of the
instruction that moves a value into a predicate register, we might be
able to lower this limit a bit more in the future, but since we need 6
cycles from the move to predicate register to predicated branch, that
puts some sort of lower bound on how far we can lower this threshold.

Signed-off-by: Rob Clark <robdclark@gmail.com>