nir/search: Allow for matching variables based on types

nir/search: Allow for matching variables based on types

This allows you to match on an unknown value but only if it is of a given
type.  90% of the uses of this are for matching only booleans, but adding
the generality of arbitrary types is no more complex.

nir_algebraic.py doesn't handle this yet but that's ok because the C
language will ensure that the default type on all variables is void.

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