nir: Add an expression matching framework

This framework provides a simple way to do simple search-and-replace
operations on NIR code.  The nir_search.h header provides four simple data
structures for representing expressions:  nir_value and four subtypes:
nir_variable, nir_constant, and nir_expression.  An expression tree can
then be represented by nesting these data structures as needed.  The
nir_replace_instr function takes an instruction, an expression, and a
value; if the instruction matches the expression, it is replaced with a new
chain of instructions to generate the given replacement value.  The
framework keeps track of swizzles on sources and automatically generates
the currect swizzles for the replacement value.

Reviewed-by: Connor Abbott <[email protected]>

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