nir/algebraic: Optimize comparisons and up-casts

nir/algebraic: Optimize comparisons and up-casts

These seem like obvious enough optimizations in the world of multiple
integer bit sizes.  The only known thing which hits these at the moment
is some Vulkan CTS tests for 16-bit SSBO values which like to up-cast
and check for equality.  However, it's something that's bound to come up
as we start seeing more integers in shaders.

The optimizations of comparisons of casted values with constants are
something which we would ideally do with range analysis.  However,
lacking that, we can do it in opt_algebraic as long as one side is a
constant.

In dEQP-VK.ssbo.phys.layout.random.16bit.scalar.13, this commit, along
with the previous commit, reduce the number of instructions emitted on
Skylake from 55328 to 44546, a reduction of 20%.

Acked-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>