tree-optimization/114231 - use patterns for BB SLP discovery root stmts

tree-optimization/114231 - use patterns for BB SLP discovery root stmts

The following makes sure to use recognized patterns when vectorizing
roots during BB SLP discovery.  We need to apply those late since
during root discovery we've not yet done pattern recognition.
All parts of the vectorizer assume patterns get used, for the testcase
we mix this up when doing live lane computation.

	PR tree-optimization/114231
	* tree-vect-slp.cc (vect_analyze_slp): Lookup patterns when
	processing a BB SLP root.

	* gcc.dg/vect/pr114231.c: New testcase.