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

[Luke Lau via llvm-commits]

lukel97 wrote:

> > 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.
> 
> I think #65916 and #66211 are two different fixes for the same problem. Do you #65916 also fix the crash? Or do you really need #66211?

Both fix the crash, but I closed #65916 in favour of #66211 since my understanding of LiveIntervals is limited

https://github.com/llvm/llvm-project/pull/65997