nir: Add infastructure for generating algebraic transformation passes

This commit builds on the nir_search.h infastructure by adding a bit of
python code that makes it stupid easy to write an algebraic transformation
pass.  The nir_algebraic.py file contains four python classes that
correspond directly to the datastructures in nir_search.c and allow you to
easily generate the C code to represent them.  Given a list of
search-and-replace operations, it can then generate a function that applies
those transformations to a shader.

The transformations can be specified manually, or they can be specified
using nested tuples.  The nested tuples make a neat little language for
specifying expression trees and search-and-replace operations in a very
readable and easy-to-edit fasion.

The generated code is also fairly efficient.  Insteady of blindly calling
nir_replace_instr with every single transformation and on every single
instruction, it uses a switch statement on the instruction opcode to do a
first-order culling and only calls nir_replace_instr if the opcode is known
to match the first opcode in the search expression.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

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