vect: Use sdot for a fallback implementation of usdot

vect: Use sdot for a fallback implementation of usdot

Following a suggestion from Tamar, this patch adds a fallback
implementation of usdot using sdot.  Specifically, for 8-bit
input types:

   acc_2 = DOT_PROD_EXPR <a_unsigned, b_signed, acc_1>;

becomes:

   tmp_1 = DOT_PROD_EXPR <64, b_signed, acc_1>;
   tmp_2 = DOT_PROD_EXPR <64, b_signed, tmp_1>;
   acc_2 = DOT_PROD_EXPR <a_unsigned - 128, b_signed, tmp_2>;

on the basis that (x-128)*y + 64*y + 64*y.  Doing the two 64*y
operations first should give more time for x to be calculated,
on the off chance that that's useful.

gcc/
	* tree-vect-patterns.cc (vect_convert_input): Expect the input
	type to be signed for optab_vector_mixed_sign.  Update the vectype
	at the same time as type.
	(vect_recog_dot_prod_pattern): Update accordingly.  If usdot isn't
	available, try sdot instead.
	* tree-vect-loop.cc (vect_is_emulated_mixed_dot_prod): New function.
	(vect_model_reduction_cost): Model the cost of implementing usdot
	using sdot.
	(vectorizable_reduction): Likewise.  Skip target support test
	for lane reductions.
	(vect_emulate_mixed_dot_prod): New function.
	(vect_transform_reduction): Use it to emulate usdot via sdot.

gcc/testsuite/
	* gcc.dg/vect/vect-reduc-dot-9.c: Reduce target requirements
	from i8mm to dotprod.
	* gcc.dg/vect/vect-reduc-dot-10.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-11.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-12.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-13.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-14.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-15.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-16.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-17.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-18.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-19.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-20.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-21.c: Likewise.
	* gcc.dg/vect/vect-reduc-dot-22.c: Likewise.