[memprof] Migrate away from PointerUnion::dyn_cast (NFC) (#124505)

Note that PointerUnion::dyn_cast has 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>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses cast
because we know which alternative to expect in the ternary expression.

Author: kazutakahirata

llvm/lib/Transforms/IPO/MemProfContextDisambiguation.cpp

@@ -3598,10 +3598,9 @@ IndexCallsiteContextGraph::cloneFunctionForCallsite(
   // The next clone number is the current size of versions array.
   // Confirm this matches the CloneNo provided by the caller, which is based on
   // the number of function clones we have.
-  assert(CloneNo ==
-         (isa<AllocInfo *>(Call.call())
-              ? Call.call().dyn_cast<AllocInfo *>()->Versions.size()
-              : Call.call().dyn_cast<CallsiteInfo *>()->Clones.size()));
+  assert(CloneNo == (isa<AllocInfo *>(Call.call())
+                         ? cast<AllocInfo *>(Call.call())->Versions.size()
+                         : cast<CallsiteInfo *>(Call.call())->Clones.size()));
   // Walk all the instructions in this function. Create a new version for
   // each (by adding an entry to the Versions/Clones summary array), and copy
   // over the version being called for the function clone being cloned here.