[llvm] [IR2Vec] Initial infrastructure for MIR2Vec (PR #161463)

[Peter Collingbourne via llvm-commits]

pcc wrote:

Can you please make it so that `reference_x86_vocab_print.txt` does not need to be updated every time the list of MIR opcodes changes? For example, you could make it so that `-print-mir2vec-vocab` only prints a line if the numbers are not `0.00 0.00`.

https://github.com/llvm/llvm-project/pull/161463