tcg: Use tcg_target_available_regs in tcg_reg_alloc_mov

The move opcodes are special in that their constraints must cover
all available registers.  So instead of checking the constraints,
just use the available registers.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg: Remove unreachable code in tcg_out_op and op_defs

The INDEX_op_call case has just been obsoleted; the mov and movi
cases have not been reachable for years.  Attempt to document this
both in each tcg_out_op switch, and via TCG_OPF_NOT_PRESENT.

Because of the TCG_OPF_NOT_PRESENT change, this must be done for
all targets in a single commit.

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