[InstCombine] `xor` reduction w/ i1 elt type is a parity check

For i1 element type, `xor` and `add` are interchangeable
(https://alive2.llvm.org/ce/z/e77hhQ), so we should treat it just like
an `add` reduction and consistently transform them both:
https://alive2.llvm.org/ce/z/MjCm5W (self)
https://alive2.llvm.org/ce/z/kgqF4M (skipped zext)
https://alive2.llvm.org/ce/z/pgy3HP (skipped sext)

Though, let's emit the IR that is similar to the one we produce for
`vector_reduce_add(<n x i1>)`.

See https://bugs.llvm.org/show_bug.cgi?id=51259