Fix SLP scalar costing with stmts also used in externals

When we have the situation of an external SLP node that is
permuted the scalar stmts recorded in the permute node do not
mean the scalar computation can be removed.  We are removing
those stmts from the vectorized_scalar_stmts for this reason
but we fail to check this set when we cost scalar stmts.  Note
vectorized_scalar_stmts isn't a complete set so also pass
scalar_stmts_in_externs and check that.

The following fixes this.

This shows in PR115777 when we avoid vectorizing the load, but
on it's own doesn't help the PR yet.

	PR tree-optimization/115777
	* tree-vect-slp.cc (vect_bb_slp_scalar_cost): Do not
	cost a scalar stmt that needs to be preserved.