target/sh4: Tidy misc illegal insn checks

Now that we have a do_illegal label, use goto in order
to self-document the forcing of the exception.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20170718200255.31647-22-rth@twiddle.net>
[aurel32: fix whitespace issues]
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
