[WinEHPrepare] Update demotion logic

Summary:
Update the demotion logic in WinEHPrepare to avoid creating new cleanups by
walking predecessors as necessary to insert stores for EH-pad PHIs.

Also avoid creating stores for EH-pad PHIs that have no uses.

The store/load placement is still pretty naive.  Likely future improvements
(at least for optimized compiles) include:
 - Share loads for related uses as possible
 - Coalesce non-interfering use/def-related PHIs
 - Store at definition point rather than each PHI pred for non-interfering
   lifetimes.


Reviewers: rnk, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11955

llvm-svn: 244894