glsl: Set the precision of function return value temporaries.

The signature should dictate the precision of the temp we store into.
This ends up ignored by lower_precision for now, which always rewrites it
so as to handle custom lowering of builtin precision..

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21666>

src/compiler/glsl/ast_function.cpp

@@ -651,6 +651,7 @@ generate_call(exec_list *instructions, ir_function_signature *sig,
       ir_variable *var;
 
       var = new(ctx) ir_variable(sig->return_type, name, ir_var_temporary);
+      var->data.precision = sig->return_precision;
       instructions->push_tail(var);
 
       ralloc_free(name);