WebAssembly: handle `ret void`.

Summary:
Use -1 as numoperands for the return SDTypeProfile, denoting that return is variadic. Note that the patterns in InstrControl.td still need to match the inputs, so this ins't an "anything goes" variadic on ret!

The next step will be to handle other local types (not just int32).

Reviewers: sunfish

Subscribers: llvm-commits, jfb

Differential Revision: http://reviews.llvm.org/D11692

llvm-svn: 243783