target/openrisc: Cache constant 0 in DisasContext

target/openrisc: Cache constant 0 in DisasContext

We are virtually certain to have fetched constant 0 once, at the
beginning of the TB, so we might as well use it elsewhere.

Reviewed-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>