[llvm] CodeGen: Do not store RegisterClass copy costs as a signed value (PR #161786)

[Mikael Holmen via llvm-commits]

mikaelholmen wrote:

I've verified that this patch solves at least one of the regressions we saw with https://github.com/llvm/llvm-project/pull/160084

https://github.com/llvm/llvm-project/pull/161786