[WebAssembly] Don't modify preds/succs iterators while erasing from them

Summary:
This caused out-of-bound bugs. Found by
`-DLLVM_ENABLE_EXPENSIVE_CHECKS=ON`.

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D52902

llvm-svn: 343814