nir: add lower_ftrunc

Port TGSI TRUNC lowering to nir

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2246,6 +2246,8 @@ typedef struct nir_shader_compiler_options {
    /** lowers fceil to fneg+ffloor+fneg: */
    bool lower_fceil;
 
+   bool lower_ftrunc;
+
    bool lower_ldexp;
 
    bool lower_pack_half_2x16;