[NFC][InstCombine] `vector_reduce_xor(?ext(<n x i1>))` --> `?ext(vector_reduce_add(<n x i1>))`

Instead of expanding it ourselves,
we can just forward to `?ext(vector_reduce_add(<n x i1>))`, as per alive2:
https://alive2.llvm.org/ce/z/ymz7zE (self)
https://alive2.llvm.org/ce/z/eKu2v2 (skipped zext)
https://alive2.llvm.org/ce/z/c3BXgc (skipped sext)