tcg/optimize: Split out fold_ix_to_i

Pull the "op r, 0, b => movi r, 0" optimization into a function,
and use it in fold_shift.

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