[llvm] [GISEL] Add G_SPLAT_VECTOR_PARTS to represent 64-bit splat vectors on… (PR #86970)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> You can check `AArch64InstrGISel.td` for examples of where we define our target specific G_ opcodes. That said if you do this please add `RISCV` to the name like `G_RISCV_SPLAT_VECTOR`. We need to rename the AArch64 ones too.

I thought that these opcodes are part of the RISCV namespace. Do they need to duplicate RISCV in the name too?

https://github.com/llvm/llvm-project/pull/86970