[WebAssembly][MC] Fix missing `else` after `return` due to type checker bug

Once we are in the `Unreachable` we want to disable type checking, but
we were unconditionally returning `true` here which means we encountered
and error.  Instead we unconditionally return false to signal no error.

Fixes: https://github.com/llvm/llvm-project/issues/56935

Differential Revision: https://reviews.llvm.org/D135195