vect: Fix infinite loop while determining peeling amount

This is a follow up to commit 5c9669a0 respectively discussion
https://gcc.gnu.org/pipermail/gcc-patches/2020-June/549132.html

In case a type has a lower alignment than its size, ensure that we
advance.  For example, on s390x we have for a long double an alignment
constraint of 8 bytes whereas the size is 16 bytes.  Increasing the loop
variable only by TARGET_ALIGN / DR_SIZE which equals zero in case of a
long double results in an infinite loop.  This is fixed by ensuring that
we advance at least by one.

gcc/ChangeLog:

	* tree-vect-data-refs.c (vect_enhance_data_refs_alignment):
	Ensure that loop variable npeel_tmp advances in each iteration.