[DAGCombiner] Fold vecreduce_or/and if operand is insert_subvector.

Fold:
  vecreduce_or(insert_subvec(zeroinitializer, vec))
  -> vecreduce_or(vec)

  vecreduce_and(insert_subvec(allones, vec))
  -> vecreduce_and(vec)

  vecreduce_and/or(insert_subvec(undef, vec))
  -> vecreduce_and/or(vec)

This is useful for SVE which uses insert/extract subvector
to convert fixed-width to/from scalable vectors.

Reviewed By: bsmith

Differential Revision: https://reviews.llvm.org/D118919