WebAssembly: don't omit dead vregs from locals

Summary:
This is a temporary hack until we get around to remapping the vreg
numbers to local numbers. Dead vregs cause bad numbering and make
consumers sad.

We could also just look at debug info an use named locals instead, but
vregs have to work properly anyways so there!

Reviewers: binji, sunfish

Subscribers: jfb, llvm-commits, dschuff

Differential Revision: http://reviews.llvm.org/D13839

llvm-svn: 250594