[RDA] Don't adjust ReachingDefDefaultVal (NFCI)

[nikic]

Summary

At the end of a basic block, RDA adjusts all the reaching defs it found to be relative to the end of the basic block, rather than the start of it. However, it also does this to registers which don't have a reaching def, indicated by ReachingDefDefaultVal. This means that at the entry of a successor block, we end up not skipping these incoming reaching definitions, and insert them into the reaching definition list. This is ultimately harmless, but causes unnecessary work and is logically not right.

[samparker]

Thanks.

llvm/lib/CodeGen/ReachingDefAnalysis.cpp

void ReachingDefAnalysis::leaveBasicBlock(
   // Save register clearances at end of MBB - used by enterBasicBlock().
   MBBOutRegsInfos[MBBNumber] = LiveRegs;
 
   // While processing the basic block, we kept `Def` relative to the start
   // of the basic block for convenience. However, future use of this information
   // only cares about the clearance from the end of the block, so adjust
   // everything to be relative to the end of the basic block.
   for (int &OutLiveReg : MBBOutRegsInfos[MBBNumber])
+    if (OutLiveReg != ReachingDefDefaultVal)
       OutLiveReg -= CurInstr;
   LiveRegs.clear();
 }
 
 void ReachingDefAnalysis::processDefs(MachineInstr *MI) {
   assert(!MI->isDebugInstr() && "Won't process debug instructions");
 
   unsigned MBBNumber = MI->getParent()->getNumber();
   assert(MBBNumber < MBBReachingDefs.size() &&