st/nir: Free the GLSL IR after linking.

i965 does this, and st's tgsi path does this.  st/nir did not.

Cuts 138MB of memory from a DiRT Rally trace, which is about 44%
of the total GLSL IR memory.

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

src/mesa/state_tracker/st_glsl_to_nir.cpp

@@ -839,6 +839,10 @@ st_link_nir(struct gl_context *ctx,
       }
 
       nir_sweep(shader->Program->nir);
+
+      /* The GLSL IR won't be needed anymore. */
+      ralloc_free(shader->ir);
+      shader->ir = NULL;
    }
 
    return true;