[WebAssembly] Fix a bug deleting instruction in a ranged for loop

Summary: We shouldn't delete elements while iterating a ranged for loop.

Reviewers: dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58519

llvm-svn: 354844