[WebAssembly] Small improvements in FixIrreducibleControlFlow (NFC)

Summary:
- Make some class member methods const
- Delete unnecessary includes
- Use a simpler form of `BuildMI`

Reviewers: kripken

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59454

llvm-svn: 356440