[WebAssembly] Irreducible control flow rewrite

Summary:
Rewrite WebAssemblyFixIrreducibleControlFlow to a simpler and cleaner
design, which directly computes reachability and other properties
itself. This avoids previous complexity and bugs. (The new graph
analyses are very similar to how the Relooper algorithm would find loop
entries and so forth.)

This fixes a few bugs, including where we had a false positive and
thought fannkuch was irreducible when it was not, which made us much
larger and slower there, and a reverse bug where we missed
irreducibility. On fannkuch, we used to be 44% slower than asm2wasm and
are now 4% faster.

Reviewers: aheejin

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

Differential Revision: https://reviews.llvm.org/D58919

Patch by Alon Zakai (kripken)

llvm-svn: 356313