[WebAssembly] Move CFG-changing passes before RegStackify

Summary:
FixIrreducibleControlFlow and LateEHPrepare both possibly modify CFG and
create new registers. There seems to be no reason these passes go after
register-related optimization passes (PrepareForLiveIntervals,
OptimizeLiveIntervals, StoreResults, RegStackify, and RegColoring), and
this also possibly create new optimization opportunities. I think we
should put all current and future optimization passes before RegStackify
(and related passes) unless there's a reason not to.

Reviewers: kripken

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

Differential Revision: https://reviews.llvm.org/D56356

llvm-svn: 350596