tree-optimization/106081 - elide redundant permute

The following patch makes sure to elide a redundant permute that
can be merged with existing splats represented as load permutations
as we now do for non-grouped SLP loads.  This is the last bit
missing to fix this PR where the main fix was already done by
r14-2117-gdd86a5a69cbda4

	PR tree-optimization/106081
	* tree-vect-slp.cc (vect_optimize_slp_pass::start_choosing_layouts):
	Assign layout -1 to splats.

	* gcc.dg/vect/pr106081.c: New testcase.