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

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Note: See the second commit for the diff between the precommit and the new changes ([diff here](https://github.com/llvm/llvm-project/pull/108401/commits/8033c5caf3a98504ced4f312d843ca5a5baa3ddd#diff-31861103df84cde0c30aa05b21deefe205e90614c5e8c2ef6e8845ec5cc73f8e)).

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