[WebAssembly] Optimize Irreducible Control Flow

Summary:
Irreducible control flow is not that rare, e.g. it happens in malloc and
3 other places in the libc portions linked in to a hello world program.
This patch improves how we handle that code: it emits a br_table to
dispatch to only the minimal necessary number of blocks. This reduces
the size of malloc by 33%, and makes it comparable in size to asm2wasm's
malloc output.

Added some tests, and verified this passes the emscripten-wasm tests run
on the waterfall (binaryen2, wasmobj2, other).

Reviewers: aheejin, sunfish

Subscribers: mgrang, jgravelle-google, sbc100, dschuff, llvm-commits

Differential Revision: https://reviews.llvm.org/D55467

Patch by Alon Zakai (kripken)

llvm-svn: 350367