nir/algebraic: add a few masking-before-unpack optimizations

nir/algebraic: add a few masking-before-unpack optimizations

Helps some Dawn of War 3 and F1 2017 shaders with ACO:
Totals from affected shaders:
SGPRS: 2136 -> 2128 (-0.37 %)
VGPRS: 1624 -> 1628 (0.25 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 168068 -> 164332 (-2.22 %) bytes
LDS: 44 -> 44 (0.00 %) blocks
Max Waves: 222 -> 221 (-0.45 %)
Wait states: 0 -> 0 (0.00 %)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>