[WinEH] Disable most forms of demotion

[WinEH] Disable most forms of demotion

Now that the register allocator knows about the barriers on funclet
entry and exit, testing has shown that this is unnecessary.

We still demote PHIs on unsplittable blocks due to the differences
between the IR CFG and the Machine CFG.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253619 91177308-0d34-0410-b5e6-96231b3b80d8