Address most of Duncan's review comments. Also, make LoopInstSimplify a simple

Address most of Duncan's review comments. Also, make LoopInstSimplify a simple
FunctionPass. It probably doesn't have a reason to be a LoopPass, as it will
probably drop the simple fixed point and either use RPO iteration or Duncan's
approach in instsimplify of only revisiting instructions that have changed.

The next step is to preserve LoopSimplify. This looks like it won't be too hard,
although the pass manager doesn't actually seem to respect when non-loop passes
claim to preserve LCSSA or LoopSimplify. This will have to be fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122791 91177308-0d34-0410-b5e6-96231b3b80d8