If INDEX_op_foo is always expanded by tcg_expand_vec_op, then
there may be no reasonable set of constraints to return from
tcg_target_op_def for that opcode.

Let TCG_TARGET_HAS_foo be specified as -1 in that case.  Thus a
boolean test for TCG_TARGET_HAS_foo is true, but we will not
assert within process_op_defs when no constraints are specified.

Compare this with tcg_can_emit_vec_op, which already uses this
tri-state indication.

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