tcg: Reorg function calls

tcg: Reorg function calls

Pre-compute the function call layout for each helper at startup.
Drop TCG_CALL_DUMMY_ARG, as we no longer need to leave gaps
in the op->args[] array.  This allows several places to stop
checking for NULL TCGTemp, to which TCG_CALL_DUMMY_ARG mapped.

For tcg_gen_callN, loop over the arguments once.  Allocate the TCGOp
for the call early but delay emitting it, collecting arguments first.
This allows the argument processing loop to emit code for extensions
and have them sequenced before the call.

For tcg_reg_alloc_call, loop over the arguments in reverse order,
which allows stack slots to be filled first naturally.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>