target/hexagon: Include helper-gen.h where needed

This had been included via tcg-op-common.h via tcg-op.h,
but that is going away.  In idef-parser.y, shuffle some
tcg related includes into a more logical order.

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