[polly] [Polly] Do not invalidate SCEV before codegen (PR #194677)

[Karthika Devi C via llvm-commits]

kartcq wrote:

This fix looks fine for now, as we are also not sure of why we added code to forget values in past commit.
@Meinersbur Do you think we can get opinion from Johannes on this as well.
Or if this fix is convincing enough for you, we can proceed to merge as well.
Thanks for looking into the issue and proposing fix.

https://github.com/llvm/llvm-project/pull/194677