WebAssembly: print basic integer assembly.

Summary:
This prints assembly for int32 integer operations defined in WebAssemblyInstrInteger.td only, with major caveats:

  - The operation names are currently incorrect.
  - Other integer and floating-point types will be added later.
  - The printer isn't factored out to handle recursive AST code yet, since it can't even handle control flow anyways.
  - The assembly format isn't full s-expressions yet either, this will be added later.
  - This currently disables PrologEpilogCodeInserter as well as MachineCopyPropagation becasue they don't like virtual registers, which WebAssembly likes quite a bit. This will be fixed by factoring out NVPTX's change (currently a fork of PrologEpilogCodeInserter).

Reviewers: sunfish

Subscribers: llvm-commits, jfb

Differential Revision: http://reviews.llvm.org/D11671

llvm-svn: 243763