[CallSite removal] Add `CallBase` support to the `InstVisitor` in such a way that it still supports `CallSite` but users can be ported to rely on `CallBase` instead.

[chandlerc]

Summary

This will unblock the ports across the analysis and transforms libraries
(and out-of-tree users) and once done we can clean this up by removing
the CallSite layer.

[compnerd]

Clever!

llvm/trunk/include/llvm/IR/InstVisitor.h

#include "llvm/IR/Instruction.def"
   // of instructions...
   //
   RetTy visitCastInst(CastInst &I)                { DELEGATE(UnaryInstruction);}
   RetTy visitUnaryOperator(UnaryOperator &I)      { DELEGATE(UnaryInstruction);}
   RetTy visitBinaryOperator(BinaryOperator &I)    { DELEGATE(Instruction);}
   RetTy visitCmpInst(CmpInst &I)                  { DELEGATE(Instruction);}
   RetTy visitUnaryInstruction(UnaryInstruction &I){ DELEGATE(Instruction);}
 
-  // Provide a special visitor for a 'callsite' that visits both calls and
-  // invokes. When unimplemented, properly delegates to either the terminator or
-  // regular instruction visitor.
+  // The next level delegation for `CallBase` is slightly more complex in order
+  // to support visiting cases where the call is also a terminator.
+  RetTy visitCallBase(CallBase &I) {
+    if (isa<InvokeInst>(I))
+      return static_cast<SubClass *>(this)->visitTerminator(I);
+
+    DELEGATE(Instruction);
+  }
+
+  // Provide a legacy visitor for a 'callsite' that visits both calls and
+  // invokes.
+  //
+  // Prefer overriding the type system based `CallBase` instead.
   RetTy visitCallSite(CallSite CS) {
     assert(CS);
     Instruction &I = *CS.getInstruction();
-    if (CS.isCall())
-      DELEGATE(Instruction);
-
-    assert(CS.isInvoke());
-    return static_cast<SubClass *>(this)->visitTerminator(I);
+    DELEGATE(CallBase);
   }
 
   // If the user wants a 'default' case, they can choose to override this
   // function.  If this function is not overloaded in the user's subclass, then
   // this instruction just gets ignored.
   //
   // Note that you MUST override this function if your return type is not void.
   //