[llvm] [NVPTX] Implemented istruncatefree and iszextfree alongwith test cases. (PR #115139)

[Justin Fargnoli via llvm-commits]

https://github.com/justinfargnoli commented:

Going forward, I recommend pushing additional changes to the same PR instead of opening up a new PR for each iteration. This helps you and the reviewers ensure that all suggestions have been addressed and the code is in the best state possible! 

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Modify the title of the PR:
- istruncatefree -> `isTruncateFree(EVT, EVT)`
- Delete "alongwith test cases". It's implied that ever change will update the test cases accordingly. 

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Overall this is a great iteration! I think we're making good progress towards a PR that'll be ready to go into the codebase. 

https://github.com/llvm/llvm-project/pull/115139