This will let us choose how to interpret a given constraint
depending on whether the opcode is 32- or 64-bit.  Which will
let us share more constraint combinations between opcodes.

At the same time, change the interface to return the advanced
pointer instead of passing it in/out by reference.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>