slp: fix is_linear_load_p to prevent multiple answers

slp: fix is_linear_load_p to prevent multiple answers

This fixes an issue where is_linear_load_p could return the incorrect
permutation kind because it is singe pass.

This arranges the candidates in such a way that there won't be any ambiguity so
that the function can still be linear but give correct values.

gcc/ChangeLog:

	* tree-vect-slp-patterns.c (is_linear_load_p): Fix ambiguity.