tcg/tci: Split out tci_args_rrs

tcg/tci: Split out tci_args_rrs

Begin splitting out functions that do pure argument decode,
without actually loading values from the register set.

This means that decoding need not concern itself between
input and output registers.  We can assert that the register
number is in range during decode, so that it is safe to
simply dereference from regs[] later.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>