nir: add nir_type_conversion_op()

This function returns the nir_op corresponding to the conversion between
the given nir_alu_type arguments.

This function lacks support for integer-based types with bit_size != 32
and for float16 conversion ops.

v2:
- Improve readiness of the code and delete cases that don't happen now (Jason)

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -699,6 +699,8 @@ nir_get_nir_type_for_glsl_type(const struct glsl_type *type)
    }
 }
 
+nir_op nir_type_conversion_op(nir_alu_type src, nir_alu_type dst);
+
 typedef enum {
    NIR_OP_IS_COMMUTATIVE = (1 << 0),
    NIR_OP_IS_ASSOCIATIVE = (1 << 1),