[llvm] [MIR2Vec] Added create factory methods for Vocabulary (PR #162569)

[S. VenkataKeerthy via llvm-commits]

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svkeerthy wrote:

SetUp() method either assigns a valid value to TII or skips the test entirely. Also, TII will be destroyed when TM is destroyed automatically after each cycle right? Trying to understand if explicitly setting it to nullptr make a difference..

https://github.com/llvm/llvm-project/pull/162569