[WebAssembly] Add more utility functions

Summary:
Added more utility functions that will be used in EH-related passes Also
changed `LoopBottom` function to `getBottom` and uses templates to be
able to handle other classes as well, which will be used in CFGSort
later.

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D48262

llvm-svn: 335006