nir: add lower_all_io_to_temps flag

This will be used for freedreno and vc4 which require all inputs
and outputs to be copied to temps.

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

src/compiler/nir/nir.h

@@ -1873,6 +1873,8 @@ typedef struct nir_shader_compiler_options {
    bool lower_extract_byte;
    bool lower_extract_word;
 
+   bool lower_all_io_to_temps;
+
    /**
     * Does the driver support real 32-bit integers?  (Otherwise, integers
     * are simulated by floats.)