nir: add new constant folding infrastructure

Add a required field to the Opcode class, const_expr, that contains an
expression or statement that computes the result of the opcode given known
constant inputs. Then take those const_expr's and expand them into a function
that takes an opcode and an array of constant inputs and spits out the constant
result. This means that when adding opcodes, there's one less place to update,
and almost all the opcodes are self-documenting since the information on how to
compute the result is right next to the definition.

The helper functions in nir_constant_expressions.c were taken from
ir_constant_expressions.cpp.

v3 Jason Ekstrand <[email protected]>
 - Use mako to generate one function per opcode instead of doing piles of
   string splicing

v4 Jason Ekstrand <[email protected]>
 - More comments and better indentation in the mako
 - Add a description of the constant expression language in nir_opcodes.py
 - Added nir_constant_expressions.py to EXTRA_DIST in Makefile.am

Signed-off-by: Jason Ekstrand <[email protected]>
Reviewed-by: Connor Abbott <[email protected]>

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