tcg/optimize: Use fold_xi_to_x for mul

Recognize the identity function for low-part multiply.

Suggested-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
1 file changed