[WebAssembly] Improve readability of irreducibility tests

Summary:
This adds `preds` comment lines to BB names for readability, while also
fixes some of existing incorrect comment lines. Also deletes a few
unnecessary attributes. Autogenerated by `opt`.

Reviewers: kripken

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59456

llvm-svn: 356439