[LoopInterchange] Track all dependencies, not just anti dependencies.

Currently, we give up on loop interchange if we encounter a flow dependency
anywhere in the loop list. Worse yet, we don't even track output dependencies.

This patch updates the dependency matrix computation to track flow and output
dependencies in the same way we track anti dependencies.

This improves an internal workload by 2.2x.

Note the loop interchange pass is off by default and it can be enabled with
'-mllvm -enable-loopinterchange'

Differential Revision: https://reviews.llvm.org/D24564

llvm-svn: 282101