Support non-optimized builds on pure wasm backend

[aturon]

Currently, compilation tends to go awry if you use wasm32-unknown-unknown without optimizations enabled. This is essentially due to immaturity of the rest of the toolchain; this issue is just for tracking the status.

[alexcrichton]

I believe at least one piece in solving this will be #3. If you leave LLVM assertions turned on then at lower optimization levels it often hits an LLVM assertion. There may also be bugs in binaryen's s2wasm right now, but I don't think we've ever gotten far enough to test that.

We shouldn't close this out automatically with #3 though because it's likely we'll need to do more than just upgrade llvm.

[alexcrichton]

Since upgrading to LLVM 6 this is unfortunately still not fixed, and I've started closing bugs in favor of rust-lang/rust#46367 which I plan to use as the rustc tracking issue for this.

[alexcrichton]

Initial testing shows that https://github.com/rust-lang-nursery/rust-wasm/issues/2 plus https://github.com/rust-lang-nursery/rust-wasm/issues/3 will solve this issue. Once those are in-tree though we should go through all the linked test cases on rust-lang/rust#46367 and make sure they work.

[alexcrichton]

With #2 and #3 now landed, I believe this works!