[WebAssembly] Gather EH instructions in one place. NFC.

Summary:
- Gather EH instructions in one place for easy tracking (more will be
  added later)
- Variable name change

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D43742

llvm-svn: 326522