Failure to do so results in the tcg optimizer sign-extending
any constant fold from 32-bits.  This turns out to be visible
in the RISC-V testsuite using a host that emits these opcodes
(e.g. any non-x86_64).

Reported-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>