WebAssembly: textual emission uses expected opcode names

Summary: WebAssembly's tablegen instructions have the names WebAssembly expects, but by LLVM convention they're uppercase and suffixed with their type after an underscore. Leave the C++ code that way, but print outt he names WebAssembly expects (lowercase, no type). We could teach tablegen to do this later, maybe by using `!cast<string>(node)` in the .td files.

Reviewers: sunfish

Subscribers: jfb, llvm-commits

Differential Revision: http://reviews.llvm.org/D11776

llvm-svn: 244305