[RISCV] Fix operand number in debug message in RISCVMergeBaseOffset.

This used to print from the ADDI where the operand number was
correct. It recently changed to print from the LUI or AUIPC which
needs to use operand 1 instead of 2.

This shows up as a crash with -debug.

llvm/lib/Target/RISCV/RISCVMergeBaseOffset.cpp

@@ -426,7 +426,7 @@ bool RISCVMergeBaseOffsetOpt::runOnMachineFunction(MachineFunction &Fn) {
       if (!detectFoldable(Hi, Lo))
         continue;
       LLVM_DEBUG(dbgs() << "  Found lowered global address: "
-                        << *Hi.getOperand(2).getGlobal() << "\n");
+                        << *Hi.getOperand(1).getGlobal() << "\n");
       MadeChange |= detectAndFoldOffset(Hi, *Lo);
       MadeChange |= foldIntoMemoryOps(Hi, *Lo);
     }