[TableGen] Make OpcodeMappings sort comparator deterministic NFCI
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@354910 91177308-0d34-0410-b5e6-96231b3b80d8

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