[llvm] [RISCV] Stop passing the merge opcode around in RISCVMoveMerger. NFC (PR #153687)

[Craig Topper via llvm-commits]

topperc wrote:

This will conflict with https://github.com/llvm/llvm-project/pull/153644 which I plan to commit first.

https://github.com/llvm/llvm-project/pull/153687