slp: handle externals correctly in linear_loads_p

slp: handle externals correctly in linear_loads_p

This fixes a bug with externals and linear_loads_p where I forgot to save the
value before returning.

It also fixes handling of nodes with multiple children on a non VEC_PERM node.
There the child iteration would already resolve the kind and the loads are All
expected to be the same if valid so just return one.

gcc/ChangeLog:

	* tree-vect-slp-patterns.c (linear_loads_p): Fix externals.