[RISCV] Shrink vslideup's LMUL when lowering fixed insert_subvector (#65997)

Similar to #65598, if we're using a vslideup to insert a fixed length
vector into another vector, then we can work out the minimum number of
registers it will need to slide up across given the minimum VLEN, and
shrink the type operated on to reduce LMUL accordingly.

This is somewhat dependent on #66211 , since it introduces a subregister
copy that triggers a crash with -early-live-intervals in one of the
tests.

Stacked upon #66211