* Pull some code out into the definedInRegion/definedInCaller methods

* Pull some code out into the definedInRegion/definedInCaller methods
* Add a stub for the severSplitPHINodes which will allow us to bbextract
  bb's with PHI nodes in them soon.
* Remove unused arguments from findInputsOutputs
* Dramatically simplify the code in findInputsOutputs.  In particular,
  nothing really cares whether or not a PHI node is using something.
* Move moveCodeToFunction to after emitCallAndSwitchStatement as that's the
  order they get called.
* Fix a bug where we would code extract a region that included a call to
  vastart.  Like 'alloca', calls to vastart must stay in the function that
  they are defined in.
* Add some comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13482 91177308-0d34-0410-b5e6-96231b3b80d8