[RISCV] Initialize MergeBaseOffsetOptPass so it will work with print-before/after-all.

If its not in the PassRegistry it's not recognized as
a pass when we print before/after. Happened to notice while
I was working on a new pass.

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -37,6 +37,7 @@ extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeRISCVTarget() {
   RegisterTargetMachine<RISCVTargetMachine> Y(getTheRISCV64Target());
   auto PR = PassRegistry::getPassRegistry();
   initializeGlobalISel(*PR);
+  initializeRISCVMergeBaseOffsetOptPass(*PR);
   initializeRISCVExpandPseudoPass(*PR);
 }