[llvm] [RISCV][GISEL] Legalize G_INSERT_VECTOR_ELT (PR #108250)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

You can just directly consume the constant index case of G_INSERT_VECTOR_ELT, G_INSERT doesn't really buy anything other than pulling in the constant 

https://github.com/llvm/llvm-project/pull/108250