tree-optimization/114736 - SLP DFS walk issue

The following fixes a DFS walk issue when identifying to be ignored
latch edges.  We have (bogus) SLP_TREE_REPRESENTATIVEs for VEC_PERM
nodes so those have to be explicitly ignored as possibly being PHIs.

	PR tree-optimization/114736
	* tree-vect-slp.cc (vect_optimize_slp_pass::is_cfg_latch_edge):
	Do not consider VEC_PERM_EXPRs as PHI use.

	* gfortran.dg/vect/pr114736.f90: New testcase.