[GVN][NFC] Use more surgical updates to save some compile time

[mkazantsev]

Summary

Rather than unconditionally discard blocks in ICF and then trigger their recalculation,
we can only drop cached info when we actually change the first ICF instruction.

lib/Transforms/Scalar/GVN.cpp

for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
     bool AtStart = BI == BB->begin();
     if (!AtStart)
       --BI;
 
     for (auto *I : InstrsToErase) {
       assert(I->getParent() == BB && "Removing instruction from wrong block?");
       LLVM_DEBUG(dbgs() << "GVN removed: " << *I << '\n');
       salvageDebugInfo(*I);
+      ICF->invalidateInstruction(I);
       if (MD) MD->removeInstruction(I);
       LLVM_DEBUG(verifyRemoved(I));
       I->eraseFromParent();
     }
 
-    ICF->invalidateBlock(BB);
     InstrsToErase.clear();
 
     if (AtStart)
       BI = BB->begin();
     else
       ++BI;
   }
 
bool GVN::performScalarPRE(Instruction *CurInst) {
   removeFromLeaderTable(ValNo, CurInst, CurrentBlock);
 
   LLVM_DEBUG(dbgs() << "GVN PRE removed: " << *CurInst << '\n');
   if (MD)
     MD->removeInstruction(CurInst);
   LLVM_DEBUG(verifyRemoved(CurInst));
   // FIXME: Intended to be markInstructionForDeletion(CurInst), but it causes
   // some assertion failures.
-  ICF->invalidateBlock(CurrentBlock);
+  ICF->invalidateInstruction(CurInst);
   CurInst->eraseFromParent();
   ++NumGVNInstr;
 
   return true;
 }
 
 /// Perform a purely local form of PRE that looks for diamond
 /// control flow patterns and attempts to perform simple PRE at the join point.