[NFC] [MTE] simplify tagp logic (#110337)

We would put a placeholder in the tagp instruction, then replace all
uses of the original alloca with this, then replace the placeholder.

We use replaceUsesWithIf anyway, so it's easier to understand if we just
exclude the tagp call there.

Author: fmayer

llvm/lib/Target/AArch64/AArch64StackTagging.cpp

@@ -594,16 +594,15 @@ bool AArch64StackTagging::runOnFunction(Function &Fn) {
     IRBuilder<> IRB(Info.AI->getNextNode());
     Function *TagP = Intrinsic::getDeclaration(
         F->getParent(), Intrinsic::aarch64_tagp, {Info.AI->getType()});
-    Instruction *TagPCall =
-        IRB.CreateCall(TagP, {Constant::getNullValue(Info.AI->getType()), Base,
-                              ConstantInt::get(IRB.getInt64Ty(), Tag)});
+    Instruction *TagPCall = IRB.CreateCall(
+        TagP, {Info.AI, Base, ConstantInt::get(IRB.getInt64Ty(), Tag)});
     if (Info.AI->hasName())
       TagPCall->setName(Info.AI->getName() + ".tag");
     // Does not replace metadata, so we don't have to handle DbgVariableRecords.
     Info.AI->replaceUsesWithIf(TagPCall, [&](const Use &U) {
-      return !memtag::isLifetimeIntrinsic(U.getUser());
+      return !memtag::isLifetimeIntrinsic(U.getUser()) &&
+             U.getUser() != TagPCall;
     });
-    TagPCall->setOperand(0, Info.AI);
 
     // Calls to functions that may return twice (e.g. setjmp) confuse the
     // postdominator analysis, and will leave us to keep memory tagged after