[RISCV] Remove RISCVMergeBaseOffsetOpt from the -O0 pass pipeline.

[craig.topper]

Summary

Internally the pass skips any function with the optnone attribute. But that still requires checking each function. If the opt level is set to None we might as well just skip putting in the pipeline at all. This what is already done for many of the passes added by TargetPassConfig.

[lenary]

LGTM

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

void RISCVPassConfig::addPreEmitPass2() {
   addPass(createRISCVExpandPseudoPass());
   // Schedule the expansion of AMOs at the last possible moment, avoiding the
   // possibility for other passes to break the requirements for forward
   // progress in the LR/SC block.
   addPass(createRISCVExpandAtomicPseudoPass());
 }
 
 void RISCVPassConfig::addPreRegAlloc() {
+  if (TM->getOptLevel() != CodeGenOpt::None)
     addPass(createRISCVMergeBaseOffsetOptPass());
 }