[llvm] [RISCV] Prevent RISCVMergeBaseOffsetOpt from calling getVRegDef on a physical register. (PR #78762)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

```MachineInstr &OffsetTail = *MRI->getVRegDef(Reg);``` also appears twice in the file, don't those need guarding? Maybe hasOneUse happens to be sufficient somehow to prevent that, but if so it's not immediately obvious to me why.

https://github.com/llvm/llvm-project/pull/78762