Unbreak Windows build

[sanjoy]

Summary

Without the llvm_unreachable, Windows complains about not returning a
value from mlir::isSpeculatable on all paths.

mlir/lib/Transforms/Utils/SideEffectUtils.cpp

case Speculation::RecursivelySpeculatable:
     return true;
 
   case Speculation::Speculatable:
     return true;
 
   case Speculation::NotSpeculatable:
     return false;
   }
+
+  llvm_unreachable("Unhandled enum in mlir::isSpeculatable!");
 }