sched: fix latency of memory dependence chain edges for consistency.

For store->load dependencies that may alias, we should always use
TrueMemOrderLatency, which may eventually become a subtarget hook. In
effect, we should guarantee at least TrueMemOrderLatency on at least
one DAG path from a store to a may-alias load.

This should fix the standard mode as well as -enable-aa-sched-mi".

llvm-svn: 158380