[WebAssembly] Simplify iterator navigations (NFC)

Summary:
- Replaces some uses of `MachineFunction::iterator(MBB)` with
  `MBB->getIterator()` and `MachineBasicBlock::iterator(MI)` with
  `MI->getIterator()`, which are simpler.
- Replaces some uses of `std::prev` of `std::next` that takes a
  MachineFunction or MachineBasicBlock iterator with `getPrevNode` and
  `getNextNode`, which are also simpler.

Reviewers: sbc100

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58913

llvm-svn: 355444