amdgcn: multi-size vector reductions

Add support for vector reductions for any vector width by switching iterators
and generalising the code slightly.  There's no one-instruction way to move an
item from lane 31 to lane 0 (63, 15, 7, 3, and 1 are all fine though), and
vec_extract is probably fewer cycles anyway, so now we always reduce to an
SGPR.

gcc/ChangeLog:

	* config/gcn/gcn-valu.md (V64_SI): Delete iterator.
	(V64_DI): Likewise.
	(V64_1REG): Likewise.
	(V64_INT_1REG): Likewise.
	(V64_2REG): Likewise.
	(V64_ALL): Likewise.
	(V64_FP): Likewise.
	(reduc_<reduc_op>_scal_<mode>): Use V_ALL. Use gen_vec_extract.
	(fold_left_plus_<mode>): Use V_FP.
	(*<reduc_op>_dpp_shr_<mode>): Use V_1REG.
	(*<reduc_op>_dpp_shr_<mode>): Use V_DI.
	(*plus_carry_dpp_shr_<mode>): Use V_INT_1REG.
	(*plus_carry_in_dpp_shr_<mode>): Use V_SI.
	(*plus_carry_dpp_shr_<mode>): Use V_DI.
	(mov_from_lane63_<mode>): Delete.
	(mov_from_lane63_<mode>): Delete.
	* config/gcn/gcn.cc (gcn_expand_reduc_scalar): Support partial vectors.
	* config/gcn/gcn.md (unspec): Remove UNSPEC_MOV_FROM_LANE63.