[clang] Remove a redundant check in Mangle. NFC (#95071)

This addresses a review comment for PR #94987 Because that PR is a big
automatic change, this change was moved in a separate one.

Author: samolisov

clang/lib/AST/Mangle.cpp

@@ -301,9 +301,8 @@ void MangleContext::mangleBlock(const DeclContext *DC, const BlockDecl *BD,
   } else {
     assert((isa<NamedDecl>(DC) || isa<BlockDecl>(DC)) &&
            "expected a NamedDecl or BlockDecl");
-    if (isa<BlockDecl>(DC))
-      for (; isa_and_nonnull<BlockDecl>(DC); DC = DC->getParent())
-        (void) getBlockId(cast<BlockDecl>(DC), true);
+    for (; isa_and_nonnull<BlockDecl>(DC); DC = DC->getParent())
+      (void)getBlockId(cast<BlockDecl>(DC), true);
     assert((isa<TranslationUnitDecl>(DC) || isa<NamedDecl>(DC)) &&
            "expected a TranslationUnitDecl or a NamedDecl");
     if (const auto *CD = dyn_cast<CXXConstructorDecl>(DC))