TableGen: strengthen assert

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208679 91177308-0d34-0410-b5e6-96231b3b80d8
[un]wrap extracted from lib/Target/Target[MachineC].cpp, lib/ExecutionEngine/ExecutionEngineBindings.cpp into include/llvm/IR/DataLayout.h

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208680 91177308-0d34-0410-b5e6-96231b3b80d8