[IR] Optimize adding attribute to AttributeList (NFC)

When adding an enum attribute to an AttributeList, avoid going
through an AttrBuilder and instead directly add the attribute to
the correct set. Going through AttrBuilder is expensive, because
it requires all string attributes to be reconstructed.

This can be further improved by inserting the attribute at the
right position and using the AttributeSetNode::getSorted() API.

This recovers the small compile-time regression from D94633.

llvm/include/llvm/IR/Attributes.h

@@ -399,6 +399,9 @@ class AttributeList {
 
   static AttributeList getImpl(LLVMContext &C, ArrayRef<AttributeSet> AttrSets);
 
+  AttributeList setAttributes(LLVMContext &C, unsigned Index,
+                              AttributeSet Attrs) const;
+
 public:
   AttributeList() = default;
 

llvm/lib/IR/Attributes.cpp

@@ -1272,9 +1272,11 @@ AttributeList AttributeList::get(LLVMContext &C,
 AttributeList AttributeList::addAttribute(LLVMContext &C, unsigned Index,
                                           Attribute::AttrKind Kind) const {
   if (hasAttribute(Index, Kind)) return *this;
-  AttrBuilder B;
-  B.addAttribute(Kind);
-  return addAttributes(C, Index, B);
+  AttributeSet Attrs = getAttributes(Index);
+  // TODO: Insert at correct position and avoid sort.
+  SmallVector<Attribute, 8> NewAttrs(Attrs.begin(), Attrs.end());
+  NewAttrs.push_back(Attribute::get(C, Kind));
+  return setAttributes(C, Index, AttributeSet::get(C, NewAttrs));
 }
 
 AttributeList AttributeList::addAttribute(LLVMContext &C, unsigned Index,
@@ -1292,6 +1294,16 @@ AttributeList AttributeList::addAttribute(LLVMContext &C, unsigned Index,
   return addAttributes(C, Index, B);
 }
 
+AttributeList AttributeList::setAttributes(LLVMContext &C, unsigned Index,
+                                           AttributeSet Attrs) const {
+  Index = attrIdxToArrayIdx(Index);
+  SmallVector<AttributeSet, 4> AttrSets(this->begin(), this->end());
+  if (Index >= AttrSets.size())
+    AttrSets.resize(Index + 1);
+  AttrSets[Index] = Attrs;
+  return AttributeList::getImpl(C, AttrSets);
+}
+
 AttributeList AttributeList::addAttributes(LLVMContext &C, unsigned Index,
                                            const AttrBuilder &B) const {
   if (!B.hasAttributes())
@@ -1309,16 +1321,9 @@ AttributeList AttributeList::addAttributes(LLVMContext &C, unsigned Index,
          "Attempt to change alignment!");
 #endif
 
-  Index = attrIdxToArrayIdx(Index);
-  SmallVector<AttributeSet, 4> AttrSets(this->begin(), this->end());
-  if (Index >= AttrSets.size())
-    AttrSets.resize(Index + 1);
-
-  AttrBuilder Merged(AttrSets[Index]);
+  AttrBuilder Merged(getAttributes(Index));
   Merged.merge(B);
-  AttrSets[Index] = AttributeSet::get(C, Merged);
-
-  return getImpl(C, AttrSets);
+  return setAttributes(C, Index, AttributeSet::get(C, Merged));
 }
 
 AttributeList AttributeList::addParamAttribute(LLVMContext &C,