[llvm] [StatepointLowering] Take return attributes of `gc.result` into account (PR #68439)

[Danila Malyutin via llvm-commits]

danilaml wrote:

LGTM. Maybe add a test with signext i1 return and without any *ext attributes for completeness sake.

https://github.com/llvm/llvm-project/pull/68439