nir: add lower_ldexp to nir compiler options

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/nir/nir.h

@@ -1859,6 +1859,8 @@ typedef struct nir_shader_compiler_options {
    /** lowers ffract to fsub+ffloor: */
    bool lower_ffract;
 
+   bool lower_ldexp;
+
    bool lower_pack_half_2x16;
    bool lower_pack_unorm_2x16;
    bool lower_pack_snorm_2x16;