Revert r166390 "LoopIdiom: Replace custom dependence analysis with LoopDependenceAnalysis."

It passes all tests, produces better results than the old code but uses the
wrong pass, LoopDependenceAnalysis, which is old and unmaintained. "Why is it
still in tree?", you might ask. The answer is obviously: "To confuse developers."

Just swapping in the new dependency pass sends the pass manager into an infinte
loop, I'll try to figure out why tomorrow.

llvm-svn: 166399