[TableGen] Make OpcodeMappings sort comparator deterministic NFCI

The previous sort comparator was not deterministic, i.e. in some
situations it would be possible for lhs < rhs && rhs < lhs. This was
discovered by an STL assertion in a Windows debug build of llvm-tblgen.

Differential Revision: https://reviews.llvm.org/D58687

llvm-svn: 354910