[llvm] [GISEL][RISCV] Add G_VACOPY GISEL opcode and add lowering code for it. (PR #73066)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Can we just handle Intrinsic::vacopy in RISCVLegalizerInfo::legalizeIntrinsic like AArch64 does?

It isn't clear to me why we have G_VASTART and G_VAARG, but would use legalizeIntrinsic for vacopy. It seems fitting that all three should get opcodes or all three should use legalizeIntrinsic. We have all three ISD nodes in SDAG. We have `SDValue SelectionDAG::expandVACopy(SDNode *Node)`. This patch provides the SDAG equivalent. 

I've gone ahead and made the change and based on reviewer input we can decide which approach to take.

https://github.com/llvm/llvm-project/pull/73066