[clang][analyzer][NFC] Move dyn_cast's into if statements for readability

[steakhal]

I think should mention "NFC" for such commits. Also, we tend to use the "analyzer" tag for CSA related changes.

BTW thanks for these simplifications.

clang/lib/StaticAnalyzer/Core/MemRegion.cpp

if (R && R->canPrintPrettyAsExpr()) {
     else
       return (llvm::Twine(os.str()) + ArrayIndices).str();
   }
 
   return VariableName;
 }
 
 SourceRange MemRegion::sourceRange() const {
-  const auto *const VR = dyn_cast<VarRegion>(this->getBaseRegion());
-  const auto *const FR = dyn_cast<FieldRegion>(this);
-
   // Check for more specific regions first.
-  // FieldRegion
-  if (FR) {
+  if (auto *FR = dyn_cast<FieldRegion>(this)) {
     return FR->getDecl()->getSourceRange();
   }
-  // VarRegion
-  else if (VR) {
+
+  if (auto *VR = dyn_cast<VarRegion>(this->getBaseRegion())) {
     return VR->getDecl()->getSourceRange();
   }
+
   // Return invalid source range (can be checked by client).
-  else
-    return {};
 }
 
 //===----------------------------------------------------------------------===//
 // MemRegionManager methods.
 //===----------------------------------------------------------------------===//
 
 DefinedOrUnknownSVal MemRegionManager::getStaticSize(const MemRegion *MR,
                                                      SValBuilder &SVB) const {