target/sh4: Merge DREG into fpr64 routines

Also add a debugging assert that we did signal illegal opc
for odd double-precision registers.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20170718200255.31647-16-rth@twiddle.net>
[aurel32: fix whitespace issues]
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
