[InstCombine] When turning sext into zext due to known bits, return the new ZExt instead of calling replaceinstuseswith

[craig.topper]

Summary

The worklist loop that we're returning back to should be able to do the repacement itself. This is how we normally do replacements.

My main motivation was that I observed that we weren't preserving the name of the result when we do this transform. The replacement code in the worklist loop will call takeName as part of the replacement.

[spatel]

LGTM - there are several other transforms scattered around instcombine that produce a value with the builder and then manually replace.

These are almost NFC changes, but you can show the cosmetic diffs in regression tests by re-running 'update_test_checks.py'.

llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp

if (Instruction *I = commonCastTransforms(CI))
     return I;
 
   Value *Src = CI.getOperand(0);
   Type *SrcTy = Src->getType(), *DestTy = CI.getType();
 
   // If we know that the value being extended is positive, we can use a zext
   // instead.
   KnownBits Known = computeKnownBits(Src, 0, &CI);
-  if (Known.isNonNegative()) {
-    Value *ZExt = Builder.CreateZExt(Src, DestTy);
-    return replaceInstUsesWith(CI, ZExt);
-  }
+  if (Known.isNonNegative())
+    return CastInst::Create(Instruction::ZExt, Src, DestTy);
 
   // Try to extend the entire expression tree to the wide destination type.
   if (shouldChangeType(SrcTy, DestTy) && canEvaluateSExtd(Src, DestTy)) {
     // Okay, we can transform this!  Insert the new expression now.
     LLVM_DEBUG(
         dbgs() << "ICE: EvaluateInDifferentType converting expression type"
                   " to avoid sign extend: "
                << CI << '\n');