Type check block/loop/if sigs

[kripken]

This rewrites the type checking code to use block/if/loop sigs properly, as read in s-expr files.

This passes all the spec tests and is basically done I think. but as mentioned in #711 (comment) fails on the binary format tests on round-tripping.

This is based on #711, so it should be rebased on master after that lands.

[kripken]

This also

• Prints out if and loop sigs, which was missing and is now necessary.
• Removes the DSL in OptimizeInstructions (after these type checking changes, using wasm for the DSL became too annoying, not sure what to do there).

[dschuff]

Do we actually still need to do all the type merging now? Seems like if we give blocks their type on construction then we don't need to do anything on finalizing, and we can check break types, etc without that?

[dschuff]

Once we parse block signatures, we can just encode curr->type here which should never be unreachable. Likewise for ifs and loops.

[kripken]

Finalizing might still matter for code generators other than wasm-s-parser. But maybe that is worth refactoring later into something on the side.

[dschuff]

So the binary format difference is because in the wast parser you replaced the block finalization with the logic that instead only marks the block as unreachable if no breaks target it and the fallthrough is unreachable. If you do that in the binary parser too then it works correctly (I have that working in my local branch).

I do think the "primary" behavior should be that the type of blocks doesn't get changed by default. It might make sense to have a service for binaryen library users that might not know the block type up front (which I guess may include the optimization passes) but it doesn't need to have the same behavior.

[kripken]

Yeah, as discussed in person that's indeed the issue. I'll continue work towards that, with finalize(type) on loop/block/if using a given type (as by the s-parser or binary decoder), while finalize() with no args autodetects the type fully (which is useful in the optimizer, as you said).

[kripken]

Ok, this is hopefully good to go. Tests pass locally.