[WebAssembly] Optimize the number of routing blocks in FixIrreducibleCFG

Summary:
Currently we create a routing block to the dispatch block for every
predecessor of every entry. So the total number of routing blocks
created will be (# of preds) * (# of entries). But we don't need to do
this: we need at most 2 routing blocks per loop entry, one for when the
predecessor is inside the loop and one for it is outside the loop. (We
can't merge these into one because this will creates another loop cycle
between blocks inside and blocks outside) This patch fixes this and
creates at most 2 routing blocks per entry.

This also renames variable `Split` to `Routing`, which I think is a bit
clearer.

Reviewers: kripken

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59462

llvm-svn: 357337