[WebAssembly] Improve readability of load/stores and tests. NFC.

Summary:
- Changed variable/function names to be more consistent
- Improved comments in test files
- Added more tests
- Fixed a few typos
- Misc. cosmetic changes

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D49087

llvm-svn: 336598