[WebAssembly] Handle debug information and virtual registers without crashing

Summary: Currently, enabling debug information when compiling for WebAssembly crashes the backend. This commit fixes these by skipping debug values in backend passes.

Reviewers: jfb, aprantl, dschuff, echristo

Subscribers: mehdi_amini, yurydelendik, dexonsmith, MatzeB, jfb, dschuff, llvm-commits

Differential Revision: https://reviews.llvm.org/D21808

llvm-svn: 278967