Apply some TLC to vect_slp_analyze_instance_dependence
This refactors things, separating load and store handing, adjusting
comments to reflect reality and removing some dead code.
* tree-vect-data-refs.cc (vect_slp_analyze_store_dependences):
Split out from vect_slp_analyze_node_dependences, remove
dead code.
(vect_slp_analyze_load_dependences): Split out from
vect_slp_analyze_node_dependences, adjust comments. Process
queued stores before any disambiguation.
(vect_slp_analyze_node_dependences): Remove.
(vect_slp_analyze_instance_dependence): Adjust.