The rest of r198588. Remove SegOvrBits from X86 TSFlags since they weren't being used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198589 91177308-0d34-0410-b5e6-96231b3b80d8
MC: Fatally error if subtraction operand is bad

Instead of crashing, raise an error when a subtraction expression
involves an undefined symbol.

This fixes PR18375.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198590 91177308-0d34-0410-b5e6-96231b3b80d8