[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355444
91177308-0d34-0410-b5e6-
96231b3b80d8