[llvm] [SDAG] Avoid creating redundant stack slots when lowering FSINCOS (PR #108401)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Do we have anything else that could use this? modf comes to mind, but I don't think we have RTLIB support for that.

FFREXP has the same issue due to materializing a stack slot for the `i32` result: https://godbolt.org/z/3EK3EKMWr

https://github.com/llvm/llvm-project/pull/108401