i965/nir: Do optimizations again just before lowering source mods.

i965/nir: Do optimizations again just before lowering source mods.

We want to run CSE and algebraic optimizations again after lowering IO.
Some of the passes in the optimization loop don't handle saturates and
other modifiers, so run it before lowering to source modifiers.

total instructions in shared programs: 6046190 -> 6045768 (-0.01%)
instructions in affected programs:     22406 -> 21984 (-1.88%)
helped:                                47
HURT:                                  0
GAINED:                                0
LOST:                                  0

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>