Revert adding return types to reduction intrinsic declarations.

[m-kurtenacker]

https://github.com/llvm/llvm-project/commit/322d0afd875df66b36e4810a2b95c20a8f22ab9b changes the declaration of llvm.vector.reduce.fadd and llvm.vector.reduce.fmul to not contain the return type any more. This fixes some tests that were failing lately.