[clang-tidy][NFC] Fix tiny bug in areStatementsIdentical

Summary:
Function areStatementsIdentical had an early exit
when classes of stmt does not match. That if were
added to speed up checking and do not calculate
hash id of both objects if they are not the same
type. Due to some bug, wrong pointer were used
and this resulted with comparing self to self.

This patch fixes this issue.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

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Differential Revision: https://phabricator.intern.facebook.com/D60251054

Author: PiotrZSL

clang-tools-extra/clang-tidy/utils/ASTUtils.cpp

@@ -96,7 +96,7 @@ bool areStatementsIdentical(const Stmt *FirstStmt, const Stmt *SecondStmt,
   if (FirstStmt == SecondStmt)
     return true;
 
-  if (FirstStmt->getStmtClass() != FirstStmt->getStmtClass())
+  if (FirstStmt->getStmtClass() != SecondStmt->getStmtClass())
     return false;
 
   if (isa<Expr>(FirstStmt) && isa<Expr>(SecondStmt)) {