[NFC][InstCombine] Simplify emitted IR for `vector_reduce_xor(?ext(<n x i1>))`

Now that we canonicalize low bit splatting to the form we were emitting
here ourselves, emit simpler IR that will be canonicalized later.

See 1e801439 for proofs:
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)