[WebAssembly] Add Relooper

This is just an initial checkin of an implementation of the Relooper algorithm, in preparation for WebAssembly codegen to utilize. It doesn't do anything yet by itself.

The Relooper algorithm takes an arbitrary control flow graph and generates structured control flow from that, utilizing a helper variable when necessary to handle irreducibility. The WebAssembly backend will be able to use this in order to generate an AST for its binary format.

Author: azakai

Reviewers: jfb, sunfish

Subscribers: jevinskie, arsenm, jroelofs, llvm-commits

Differential revision: http://reviews.llvm.org/D11691

llvm-svn: 245142