[WebAssembly] Handle debug information and virtual registers without crashing (reland r278967)

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

Differential Revision: https://reviews.llvm.org/D23635

llvm-svn: 279011