[memprof] Migrate away from PointerUnion::is (NFC) (#122622)

Note that PointerUnion::is have been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

In this patch, I'm calling call().getBase() for an instance of
PointerUnion.  call() alone would return an instance of IndexCall,
which wraps PointerUnion.  Note that isa<> cannot directly accept an
instance of IndexCall, at least without defining CastInfo.

I'm not touching PointerUnion::dyn_cast for now because it's a bit
complicated; we could blindly migrate it to dyn_cast_if_present, but
we should probably use dyn_cast when the operand is known to be
non-null.

Author: kazutakahirata

llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp

@@ -3610,7 +3610,7 @@ IndexCallsiteContextGraph::cloneFunctionForCallsite(
   // Confirm this matches the CloneNo provided by the caller, which is based on
   // the number of function clones we have.
   assert(CloneNo ==
-         (Call.call().is<AllocInfo *>()
+         (isa<AllocInfo *>(Call.call().getBase())
               ? Call.call().dyn_cast<AllocInfo *>()->Versions.size()
               : Call.call().dyn_cast<CallsiteInfo *>()->Clones.size()));
   // Walk all the instructions in this function. Create a new version for