[flang] Legacy ASSIGN statement target processing (#133737)

Like other target statements, the statement associated with the label in
a legacy ASSIGN statement could be inside a construct. Constructs
containing such a target must therefore be marked as unstructured,
fairly similar to how targets are processed in `markBranchTarget`.

Author: vdonaldson

flang/lib/Lower/PFTBuilder.cpp

@@ -878,8 +878,18 @@ class PFTBuilder {
             lower::pft::Evaluation *target{
                 labelEvaluationMap->find(label)->second};
             assert(target && "missing branch target evaluation");
-            if (!target->isA<parser::FormatStmt>())
+            if (!target->isA<parser::FormatStmt>()) {
               target->isNewBlock = true;
+              for (lower::pft::Evaluation *parent = target->parentConstruct;
+                   parent; parent = parent->parentConstruct) {
+                parent->isUnstructured = true;
+                // The exit of an enclosing DO or IF construct is a new block.
+                if (parent->constructExit &&
+                    (parent->isA<parser::DoConstruct>() ||
+                     parent->isA<parser::IfConstruct>()))
+                  parent->constructExit->isNewBlock = true;
+              }
+            }
             auto iter = assignSymbolLabelMap->find(*sym);
             if (iter == assignSymbolLabelMap->end()) {
               lower::pft::LabelSet labelSet{};