[WinEHPrepare] Add rudimentary support for the new EH instructions

Summary:
This adds somewhat basic preparation functionality including:
- Formation of funclets via coloring basic blocks.
- Cloning of polychromatic blocks to ensure that funclets have unique
  program counters.
- Demotion of values used between different funclets.
- Some amount of cleanup once we have removed predecessors from basic
  blocks.
- Verification that we are left with a CFG that makes some amount of
  sense.

N.B. Arguments and numbering still need to be done.

Reviewers: rnk, JosephTremoulet

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11750

llvm-svn: 244272