[WebAssembly] Tidy up DebugValueManager (NFC)

Misc. cleanups for `WebAssemblyDebugValueManager`.
- Use `Register` for registers
- Simpler for loop iteration
- Rename a variable
- Reorder methods
- Reduce `SmallVector` size for `DBG_VALUE`s to 1; one def usually have
  a single `DBG_VALUE` attached to it in most cases
- Add a few more lines of comments

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D146743