Fix the underlying cause for the asan test failure introduced by cl/210618122.

Restored the order of deleting then-clause before else-clause in if-statement destructor, since it does not and should not matter.

PiperOrigin-RevId: 211273720

mlir/include/mlir/IR/UseDefLists.h

@@ -108,6 +108,7 @@ public:
     other.removeFromCurrent();
     value = other.value;
     other.value = nullptr;
+    other.back = nullptr;
     nextUse = nullptr;
     back = nullptr;
     insertIntoCurrent();

mlir/lib/IR/Statement.cpp

@@ -366,10 +366,11 @@ IfStmt::IfStmt(Location *location, unsigned numOperands, IntegerSet *set)
 }
 
 IfStmt::~IfStmt() {
+  delete thenClause;
+
   if (elseClause)
     delete elseClause;
 
-  delete thenClause;
   // An IfStmt's IntegerSet 'set' should not be deleted since it is
   // allocated through MLIRContext's bump pointer allocator.
 }