These match their corresponding tree codes, by taking a vector and returning a 
scalar; this is more architecturally neutral than the (somewhat loosely defined) 
previous optab that took a vector and returned a vector with the result in the 
least significant bits (i.e. element 0 for little-endian or N-1 for bigendian). 
However, the old optabs are preserved so as not to break existing backends, so 
clients check for both old + new optabs.

Bootstrap, check-gcc and check-g++ on x86_64-none-linux-gnu.
aarch64.exp + vect.exp on aarch64{,_be}-none-elf.
(of course at this point in the series all these are using the old optab + 
migration path.)

gcc/ChangeLog:

	* doc/md.texi (Standard Names): Add reduc_(plus,[us](min|max))|scal
	optabs, and note in reduc_[us](plus|min|max) to prefer the former.

	* expr.c (expand_expr_real_2): Use reduc_..._scal if available, fall
	back to old reduc_... + BIT_FIELD_REF only if not.

	* optabs.c (optab_for_tree_code): for REDUC_(MAX,MIN,PLUS)_EXPR,
	return the reduce-to-scalar (reduc_..._scal) optab.
	(scalar_reduc_to_vector): New.

	* optabs.def (reduc_smax_scal_optab, reduc_smin_scal_optab,
	reduc_plus_scal_optab, reduc_umax_scal_optab, reduc_umin_scal_optab):
	New.

	* optabs.h (scalar_reduc_to_vector): Declare.

	* tree-vect-loop.c (vectorizable_reduction): Look for optabs reducing
	to either scalar or vector.