nir: Add algebraic optimizations for pointless shifts.

nir: Add algebraic optimizations for pointless shifts.

The GLSL IR optimization pass contained these; we may as well include
them too.

v2: Fix a >> 0 and a << 0 optimizations (caught by Matt).

No change in the number of NIR instructions on a shader-db run.

total i965 instructions in shared programs: 6035397 -> 6035392 (-0.00%)
i965 instructions in affected programs:     542 -> 537 (-0.92%)
helped:                                     2 (in glamor)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>