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

[Amara Emerson via llvm-commits]

aemerson wrote:

Which is the "lower level" representation? SPLAT_VECTOR_PARTS or a SPLAT of BUILD_VECTOR? Can we lower SPLAT_VECTOR_PARTS to the build vector representation for targets that don't need it?

I'm also wondering if since only RISCV seems to need this whether we can just make it a RISCV G_ opcode.

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