[WebAssembly] Add WebAssemblyLateEHPrepare pass

Summary:
Add WebAssemblyLateEHPrepare pass that does several small jobs for
exception handling. This runs before CFGSort, and is different from
WasmEHPrepare pass that runs before ISel, even though the names are
similar.

Reviewers: dschuff, majnemer

Subscribers: sbc100, jgravelle-google, sunfish, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46803

llvm-svn: 335438