nir/algebraic: add some half packing optimizations

nir/algebraic: add some half packing optimizations

pipeline-db (ACO):
Totals from affected shaders:
SGPRS: 29200 -> 29200 (0.00 %)
VGPRS: 17372 -> 17372 (0.00 %)
Spilled SGPRs: 105 -> 105 (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: 1406576 -> 1389256 (-1.23 %) bytes
LDS: 83 -> 83 (0.00 %) blocks
Max Waves: 3976 -> 3976 (0.00 %)

pipeline-db (LLVM):
Totals from affected shaders:
SGPRS: 21320 -> 21320 (0.00 %)
VGPRS: 17056 -> 17036 (-0.12 %)
Spilled SGPRs: 22 -> 22 (0.00 %)
Spilled VGPRs: 503 -> 487 (-3.18 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 396 -> 396 (0.00 %) dwords per thread
Code Size: 1441244 -> 1423292 (-1.25 %) bytes
LDS: 463 -> 463 (0.00 %) blocks
Max Waves: 3609 -> 3611 (0.06 %)

v2: add pattern for ishr

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2271>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2271>