target/mips: Extract break code into env->error_code

Simplify cpu_loop by doing all of the decode in translate.

This fixes a bug in that cpu_loop was not handling the
different layout of the R6 version of break16.  This fixes
a bug in that cpu_loop extracted the wrong bits for the
mips16e break16 instruction.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220107213243.212806-17-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>