nir: Add opcodes to extract bytes or words.

The uint versions zero extend while the int versions sign extend.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/nir/nir.h

@@ -1471,6 +1471,9 @@ typedef struct nir_shader_compiler_options {
    bool lower_pack_half_2x16;
    bool lower_unpack_half_2x16;
 
+   bool lower_extract_byte;
+   bool lower_extract_word;
+
    /**
     * Does the driver support real 32-bit integers?  (Otherwise, integers
     * are simulated by floats.)