compiler/nir: add a lowering pass to convert the bit size of ALU operations

Not all bit-sizes may be supported natively in hardware for all operations.
This pass allows drivers to lower such operations to a bit-size that is
actually supported and then converts the result back to the original
bit-size.

Compiler backends control which operations and wich bit-sizes require
the lowering through a callback function.

v2: generalize this pass and make it available in NIR core (Rob, Jason)
v3: remove some temporaries and reduce nesting in instruction loop using
    a continue statement (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2758,6 +2758,12 @@ bool nir_lower_to_source_mods(nir_shader *shader);
 
 bool nir_lower_gs_intrinsics(nir_shader *shader);
 
+typedef unsigned (*nir_lower_bit_size_callback)(const nir_alu_instr *, void *);
+
+bool nir_lower_bit_size(nir_shader *shader,
+                        nir_lower_bit_size_callback callback,
+                        void *callback_data);
+
 typedef enum {
    nir_lower_imul64 = (1 << 0),
    nir_lower_isign64 = (1 << 1),