[DAG] Fix a miscompile in insert_subvector undef (insert_subvector undef, ..), idx combine (#73587)

The combine was implicitly assuming that the index on the outer
insert_subvector meant the same thing when the source was switched to be
the index of the inner insert_subvector. This is not true if the
innermost sub-vector is fixed, and the outer subvector is scalable.

I could do a less restrictive fix here - i.e. allow the case where the
scalability of the subvectors are the same - but there's no test
coverage which shows this transform actually has profit. Given that, go
for the simplest fix.