[DAGCombiner] fold insert_subvector of insert_subvector

This pattern:

    t33: v8i32 = insert_subvector undef:v8i32, t35, Constant:i64<0>
  t21: v16i32 = insert_subvector undef:v16i32, t33, Constant:i64<0>

...shows up in PR33758:
https://bugs.llvm.org/show_bug.cgi?id=33758
...although this patch doesn't make any difference to the final result on that yet.

In the affected tests here, it looks like it just makes RA wiggle. But we might 
as well squash this to prevent it interfering with other pattern-matching.

Differential Revision:
https://reviews.llvm.org/D56604

llvm-svn: 351008