[CallSite] Make construction from Value* (or Instruction*) explicit.

CallSite roughly behaves as a common base CallInst and InvokeInst. Bring
the behavior closer to that model by making upcasts explicit. Downcasts
remain implicit and work as before.

Following dyn_cast as a mental model checking whether a Value *V isa
CallSite now looks like this: 
  if (auto CS = CallSite(V)) // think dyn_cast
instead of:
  if (CallSite CS = V)

This is an extra token but I think it is slightly clearer. Making the
ctor explicit has the advantage of not accidentally creating nullptr
CallSites, e.g. when you pass a Value * to a function taking a CallSite
argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234601 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -1045,7 +1045,7 @@ bool MemCpyOpt::iterateOnFunction(Function &F) {
         RepeatInstruction = processMemCpy(M);
       else if (MemMoveInst *M = dyn_cast<MemMoveInst>(I))
         RepeatInstruction = processMemMove(M);
-      else if (CallSite CS = (Value*)I) {
+      else if (auto CS = CallSite(I)) {
         for (unsigned i = 0, e = CS.arg_size(); i != e; ++i)
           if (CS.isByValArgument(i))
             MadeChange |= processByValArgument(CS, i);