[NFC][ValueTracking]: Remove redundant computeKnownBits call for LoadInst in isKnownNonZero

[0xdc03]

Summary

For load instructions, computeKnownBits only checks the range metadata.
This check is already present in isKnownNonZero, so there is no need to
fall through to computeKnownBits.

This change gives a speed improvement of 0.18%:
https://llvm-compile-time-tracker.com/compare.php?from=3c6ed559e5274307995586c1499a2c8e4e0276a0&to=78b462d8c4ae079638b728c6446da5999c4ee9f8&stat=instructions:u

[nikic]

llvm/lib/Analysis/ValueTracking.cpp

2549

It would be good to move this check into the load switch case.

[0xdc03]

• Address reviewer comments

llvm/lib/Analysis/ValueTracking.cpp

2549

Done, though I do wonder why this was originally put here. I guess it is a relic from the merge of isKnownNonNull and isKnownNonZero that was never updated.

[0xdc03]

llvm/lib/Analysis/ValueTracking.cpp

2552

For that matter, I guess this could also be moved into the call instruction case?

[goldstein.w.n]

llvm/lib/Analysis/ValueTracking.cpp

2800

"handled above"? "Handled there"?

[nikic]

LGTM

llvm/lib/Analysis/ValueTracking.cpp

2552

Yes, it would make sense to move all the handling here (apart from Argument).

[0xdc03]

llvm/lib/Analysis/ValueTracking.cpp

2512

@goldstein.w.n I was referring to this.

[0xdc03]

• Rebase on main

llvm/lib/Analysis/ValueTracking.cpp

if (auto *C = dyn_cast<Constant>(V)) {
     }
 
     // For constant expressions, fall through to the Operator code below.
     if (!isa<ConstantExpr>(V))
       return false;
   }
 
   if (auto *I = dyn_cast<Instruction>(V)) {
     if (MDNode *Ranges = Q.IIQ.getMetadata(I, LLVMContext::MD_range)) {
       // If the possible ranges don't contain zero, then the value is
       // definitely non-zero.
       if (auto *Ty = dyn_cast<IntegerType>(V->getType())) {
         const APInt ZeroValue(Ty->getBitWidth(), 0);
         if (rangeMetadataExcludesValue(Ranges, ZeroValue))
           return true;
       }
     }
if (const Argument *A = dyn_cast<Argument>(V)) {
             !NullPointerIsDefined(A->getParent(), PtrTy->getAddressSpace())) ||
            A->hasNonNullAttr()))
         return true;
     }
 
     // A Load tagged with nonnull metadata is never null.
     if (const LoadInst *LI = dyn_cast<LoadInst>(V))
       if (Q.IIQ.getMetadata(LI, LLVMContext::MD_nonnull))
         return true;
 
     if (const auto *Call = dyn_cast<CallBase>(V)) {
       if (Call->isReturnNonNull())
         return true;
       if (const auto *RP = getArgumentAliasingToReturnedPointer(Call, true))
         return isKnownNonZero(RP, Depth, Q);
     }
   }
 
   if (!isa<Constant>(V) &&
       isKnownNonNullFromDominatingCondition(V, Q.CxtI, Q.DT))
if (const auto *EEI = dyn_cast<ExtractElementInst>(V)) {
         return isKnownNonZero(Vec, DemandedVecElts, Depth, Q);
       }
     }
     break;
   case Instruction::Freeze:
     return isKnownNonZero(I->getOperand(0), Depth, Q) &&
            isGuaranteedNotToBePoison(I->getOperand(0), Q.AC, Q.CxtI, Q.DT,
                                      Depth);
+  case Instruction::Load:
+    // Already checked above (see range metadata handling).
+    return false;
   case Instruction::Call:
     if (auto *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       case Intrinsic::sshl_sat:
       case Intrinsic::ushl_sat:
       case Intrinsic::abs:
       case Intrinsic::bitreverse:
       case Intrinsic::bswap:
       case Intrinsic::ctpop:
         return isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q);
       case Intrinsic::ssub_sat:
         return isNonZeroSub(DemandedElts, Depth, Q, BitWidth,