st/mesa: add ST_DEBUG=nir to print NIR shaders

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/mesa/state_tracker/st_debug.c

@@ -47,6 +47,7 @@ int ST_DEBUG = 0;
 static const struct debug_named_value st_debug_flags[] = {
    { "mesa",     DEBUG_MESA, NULL },
    { "tgsi",     DEBUG_PRINT_IR, NULL },
+   { "nir",      DEBUG_PRINT_IR, NULL },
    { "constants",DEBUG_CONSTANTS, NULL },
    { "pipe",     DEBUG_PIPE, NULL },
    { "tex",      DEBUG_TEX, NULL },