LoopIdiom: Replace custom dependence analysis with DependenceAnalysis.

Requires a lot less code and complexity on loop-idiom's side and the more
precise analysis can catch more cases, like the one I included as a test case.
This also fixes the edge-case miscompilation from PR9481.

Compile time performance seems to be slightly worse, but this is mostly due
to an extra LCSSA run scheduled by the PassManager and should be fixed there.

llvm-svn: 166874