[WebAssembly] Fix type index block type handling in type checker

The current code is
```
      ExpectBlockType = false;
      TC.setLastSig(*Signature.get());
      if (ExpectBlockType)
        NestingStack.back().Sig = *Signature.get();
```

Because of the first line, the third line's `if (ExpectBlockType)` is
always false and we don't get to update `NestingStack.back().Sig`. This
results in not correctly erroring out when the types of remaining values
on the stack do not match the block type if the block type is written in
the form of a function type. We should set `ExpectBlockType` to false
after the `if`.

Reviewed By: sbc100

Differential Revision: https://reviews.llvm.org/D147837