[llvm-ir2vec] vocab read refactor (#177837)
Modifying llvm-ir2vec vocab reading pipeline to use Vocabulary::fromFile
instead of a full pass invocation
Addresses - [comment](
https://github.com/llvm/llvm-project/pull/177092#pullrequestreview-3687563016)
and is a follow up of the patch
[here](https://github.com/llvm/llvm-project/pull/177348)
