LTO in wasm backend not that effective

E.g. on Box2D it increases code size by 2.5%, and users report similar things on the mailing list - results with fastcomp used to be better. (tested with `-flto` at compile and `-flto --llvm-lto 1` at link)

Perhaps LLVM LTO has changed a lot between the LLVM versions, and no longer shrinks code size as much? Or perhaps we are not doing something right? cc @sbc100