nir/search: Add support for matching unknown constants

nir/search: Add support for matching unknown constants

There are some algebraic transformations that we want to do but only if
certain things are constants.  For instance, we may want to replace
a * (b + c) with (a * b) + (a * c) as long as a and either b or c is constant.
While this generates more instructions, some of it will get constant
folded.

nir_algebraic.py doesn't handle this yet, but that's ok because the C
language will make sure that false is the default for now.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>