[WebAssembly] support "return" and unreachable code in asm type checker

To support return (it not being supported well was the ground cause for
https://github.com/WebAssembly/wasi-sdk/issues/200) we also have to have
at least a basic notion of unreachable, which in this case just means to stop
type checking until there is an end_block (an incoming control flow edge).
This is conservative (may miss on some type checking opportunities) but is
simple and an improvement over what we had before.

Differential Revision: https://reviews.llvm.org/D112953