tcg: Specify optional vector requirements with a list

tcg: Specify optional vector requirements with a list

Replace the single opcode in .opc with a null-terminated
array in .opt_opc.  We still require that all opcodes be
used with the same .vece.

Validate the contents of this list with CONFIG_DEBUG_TCG.
All tcg_gen_*_vec functions will check any list active
during .fniv expansion.  Swap the active list in and out
as we expand other opcodes, or take control away from the
front-end function.

Convert all existing vector aware front ends.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>