target/hppa: Free some temps in do_sub

target/hppa: Free some temps in do_sub

Two temps allocated but not freed.  Do enough subtractions
within a single TB and one can run out of temps entirely.

Fixes: b2167459ae ("target-hppa: Implement basic arithmetic")
Buglink: https://bugs.launchpad.net/qemu/+bug/1880287
Tested-by: Sven Schnelle <svens@stackframe.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200720174039.517902-1-richard.henderson@linaro.org>