Get rid of `undef` and use `poison`

[bcardosolopes]

We have been getting PR warnings such as #1701 (comment), we should move out existing uses of undef as much as possible.

Originally posted by @bcardosolopes in #1701 (comment)

[saeitoshi-10]

hi @bcardosolopes i am interested in contributing to this issue. May i know about any prerequisites that might be required before tackling this issue. I am going through the repo currently for the same

[bcardosolopes]

@saeitoshi-10 great! I'd find any places where ClangIR uses undef when lowering to LLVM, check whether traditional codegen generated poison instead, and based on those cases fix what's needed.

[bruteforceboy]

@saeitoshi-10 are you still interested in this issue? If no, I can pick it up.