vect: Ensure reduc_inputs always have vectype

vect: Ensure reduc_inputs always have vectype

Vector reduction accumulators can differ in signedness from the
final scalar result.  The conversions to handle that case were
distributed through vect_create_epilog_for_reduction; this patch
does the conversion up-front instead.

gcc/
	* tree-vect-loop.c (vect_create_epilog_for_reduction): Convert
	the phi results to vectype after creating them.  Remove later
	conversion code that thus becomes redundant.