BBVectorize: Use target costs for incoming and outgoing values instead of the depth...

BBVectorize: Use target costs for incoming and outgoing values instead of the depth heuristic.

When target cost information is available, compute explicit costs of inserting and
extracting values from vectors. At this point, all costs are estimated using the
target information, and the chain-depth heuristic is not needed. As a result, it is now, by
default, disabled when using target costs.

llvm-svn: 167256