nir: Support lowering vote intrinsics

... trivially (as allowed by the spec!) by reusing the existing
nir_opt_intrinsics code.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir.h

@@ -1821,6 +1821,8 @@ typedef struct nir_shader_compiler_options {
    bool lower_extract_byte;
    bool lower_extract_word;
 
+   bool lower_vote_trivial;
+
    /**
     * Does the driver support real 32-bit integers?  (Otherwise, integers
     * are simulated by floats.)