[llvm] [RISCV][GISEL] Legalize G_INSERT_SUBVECTOR (PR #108859)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I've updated this patch based on the discussions we had during the legalization of EXTRACT_SUBVECTOR. Now ready for review :)

https://github.com/llvm/llvm-project/pull/108859