Vector related arith missing `nsw` on addition

[bcardosolopes]

If we look at how LLVM currently codegen for this, we get a add nsw instead of a plain add, see https://godbolt.org/z/xdevh38r8. There are probably other differences too.

Originally posted by @bcardosolopes in https://github.com/llvm/clangir/pull/613#discussion_r1630329735