[WinEH] Demote values and phis live across exception handlers up front

In particular, this handles SSA values that are live *out* of a handler.
The existing code only handles values that are live *in* to a handler.

It also handles phi nodes in the block where normal control should
resume after the end of a catch handler.  When EH return points have phi
nodes, we need to split the return edge. It is impossible for phi
elimination to emit copies in the previous block if that block gets
outlined. The indirectbr that we leave in the function is only notional,
and is eliminated from the MachineFunction CFG early on.

Reviewers: majnemer, andrew.w.kaylor

Differential Revision: http://reviews.llvm.org/D9158

llvm-svn: 235545