[PtrUseVisitor] Allow using Argument as a starting point

Argument is another possible starting point for the pointer traversal,
and PtrUseVisitor should be able to handle it.

Author: Artem-B

llvm/include/llvm/Analysis/PtrUseVisitor.h

@@ -158,7 +158,7 @@ class PtrUseVisitorBase {
   ///
   /// This will visit the users with the same offset of the current visit
   /// (including an unknown offset if that is the current state).
-  void enqueueUsers(Instruction &I);
+  void enqueueUsers(Value &I);
 
   /// Walk the operands of a GEP and adjust the offset as appropriate.
   ///
@@ -209,11 +209,14 @@ class PtrUseVisitor : protected InstVisitor<DerivedT>,
 
   /// Recursively visit the uses of the given pointer.
   /// \returns An info struct about the pointer. See \c PtrInfo for details.
-  PtrInfo visitPtr(Instruction &I) {
+  /// We may also need to process Argument pointers, so the input uses is
+  /// a common Value type.
+  PtrInfo visitPtr(Value &I) {
     // This must be a pointer type. Get an integer type suitable to hold
     // offsets on this pointer.
     // FIXME: Support a vector of pointers.
     assert(I.getType()->isPointerTy());
+    assert(isa<Instruction>(I) || isa<Argument>(I));
     IntegerType *IntIdxTy = cast<IntegerType>(DL.getIndexType(I.getType()));
     IsOffsetKnown = true;
     Offset = APInt(IntIdxTy->getBitWidth(), 0);

llvm/lib/Analysis/PtrUseVisitor.cpp

@@ -17,7 +17,7 @@
 
 using namespace llvm;
 
-void detail::PtrUseVisitorBase::enqueueUsers(Instruction &I) {
+void detail::PtrUseVisitorBase::enqueueUsers(Value &I) {
   for (Use &U : I.uses()) {
     if (VisitedUses.insert(&U).second) {
       UseToVisit NewU = {