Improve virtual register handling when computing debug information

Summary: Some backends, like WebAssembly, use virtual registers instead of physical registers. This crashes the DbgValueHistoryCalculator pass, which assumes that all registers are physical. Instead, skip virtual registers when iterating aliases, and assume that they are clobbered.

Reviewers: dexonsmith, dschuff, aprantl

Subscribers: yurydelendik, llvm-commits, jfb, sunfish

Differential Revision: https://reviews.llvm.org/D22590

llvm-svn: 278371