vect: Use start value in vect_load_perm_consecutive_p [PR122797].

vect: Use start value in vect_load_perm_consecutive_p [PR122797].

vect_load_perm_consecutive_p is used in a spot where we want to check
that a permutation is consecutive and starting with 0.  Originally I
wanted to add this way of checking to the function but what I ended
up with is checking whether the given permutation is consecutive
starting from a certain index.  Thus, we will return true for
e.g. {1, 2, 3} which doesn't make sense in the context of the PR.
This patch corrects it.

	PR tree-optimization/122797

gcc/ChangeLog:

	* tree-vect-slp.cc (vect_load_perm_consecutive_p): Check
	permutation start at element 0 with value instead of starting
	at a given element.
	(vect_optimize_slp_pass::remove_redundant_permutations):
	Use start value of 0.
	* tree-vectorizer.h (vect_load_perm_consecutive_p): Set default
	value to to UINT_MAX.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/pr122797.c: New test.