[WebAssembly] Don't use range-based loop for a list that's being modified

[WebAssembly] Don't use range-based loop for a list that's being modified

The first instruction in a block is what the rend() iterator points to, so
if it moves, we need to re-evaluate rend() so that we continue to iterate
through the rest of the instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256953 91177308-0d34-0410-b5e6-96231b3b80d8