[llvm] [NVPTX] Remove `NVPTX::IMAD` opcode, and rely on intruction selection only (PR #121724)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

Adding @AlexMaclean and @Artem-B as reviewers in case I'm unaware of a good reason for `NVPTXISD::IMAD` to exist. 

https://github.com/llvm/llvm-project/pull/121724