target/sh4: Consolidate end-of-TB tests

We can fold 3 different tests within the decode loop
into a more accurate computation of max_insns to start.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20170718200255.31647-3-rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
