WebAssembly: add some TODO

[sunfish]

lgtm

llvm/trunk/lib/Target/WebAssembly/README.txt

 //===-- README.txt - Notes for WebAssembly code gen -----------------------===//
 
 This WebAssembly backend is presently in a very early stage of development.
 The code should build and not break anything else, but don't expect a lot more
 at this point.
 
 For more information on WebAssembly itself, see the design documents:
   * https://github.com/WebAssembly/design/blob/master/README.md
 
 The following documents contain some information on the planned semantics and
 binary encoding of WebAssembly itself:
   * https://github.com/WebAssembly/design/blob/master/AstSemantics.md
   * https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md
 
+Interesting work that remains to be done:
+* Write a pass to restructurize irreducible control flow. This needs to be done
+  before register allocation to be efficient, because it may duplicate basic
+  blocks and WebAssembly performs register allocation at a whole-function
+  level. Note that LLVM's GPU code has such a pass, but it linearizes control
+  flow (e.g. both sides of branches execute and are masked) which is undesirable
+  for WebAssembly.
+* Basic relooper to expose control flow as an AST.
+* Figure out how to properly use MC for virtual ISAs. This may require some
+  refactoring of MC.
+
 //===---------------------------------------------------------------------===//