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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> 
This node exists in SelectionDAG because type legalization and op legalization are separate steps. That's not true in GISel where there's only one legalizer. There's no G_SPLAT_VECTOR_PARTS instruction in RISC-V. So if the legalizer in GISel creates a G_SPLAT_VECTOR_PARTS when does it get removed?

Do you think it makes more sense to lower G_SPLAT_VECTOR with i64 elements to a RISCV::G_SPLAT_VECTOR_PARTS_I64 directly?

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