tcg-aarch64: Simplify tcg_out_ldst_9 encoding

At first glance the code appears to be using 1's compliment encoding,
a-la AArch32.  Except that the constant is "off", creating a complicated
split field 2's compliment encoding.

Much clearer to just use a normal mask and shift.

Signed-off-by: Richard Henderson <rth@twiddle.net>