[flang] Fix optional assertion in PFTBuilder

[clementval]

Summary

D142279 enabled assertion in libstdc++ and one was triggered
in the PFTBuilder because an optional was access even if it was
null.
This patch fix this issue and add a regression test.

[jeanPerier]

LGTM. I think you can merge now to fix the build bot, but I would still welcome @vdonaldson feedback to asses if it is expected that targetEval.label be null in that case.

[clementval]

Done. @vdonaldson Do we need a bigger fix?

[vdonaldson]

This is a good permanent fix for the problem.

(The bug was introduced when the initial implementation that looked for GotoStatement's was extended to also look for CycleStmt's.)

Thanks @clementval for the fix.

flang/lib/Lower/PFTBuilder.cpp

for (auto it = evaluationList.begin(), end = evaluationList.end();
       auto firstStmt = [](lower::pft::Evaluation *e) {
         return e->isConstruct() ? &*e->evaluationList->begin() : e;
       };
       const Fortran::lower::pft::Evaluation &targetEval = *firstStmt(&eval);
       bool targetEvalIsEndDoStmt = targetEval.isA<parser::EndDoStmt>();
       auto branchTargetMatch = [&]() {
         if (const parser::Label targetLabel =
                 ifCandidateStack.back().ifTargetLabel)
-          if (targetLabel == *targetEval.label)
+          if (targetEval.label && targetLabel == *targetEval.label)
             return true; // goto target match
         if (targetEvalIsEndDoStmt && ifCandidateStack.back().isCycleStmt)
           return true; // cycle target match
         return false;
       };
       if (targetEval.label || targetEvalIsEndDoStmt) {
         while (!ifCandidateStack.empty() && branchTargetMatch()) {
           lower::pft::EvaluationList::iterator ifConstructIt =

flang/test/Lower/pre-fir-tree08.f

+! RUN: %flang_fc1 -fdebug-pre-fir-tree -fopenacc %s | FileCheck %s
+  program rewrite_goto
+  integer b
+  
+  b = dummy(10)
+
+  end
+      function dummy(a)
+      integer, a
+      
+      do 10 i=1,10
+  10  if(i .EQ. 1) GOTO 11
+      i=0
+  11  dummy = a + i
+      return
+      end
+
+! CHECK: <<IfConstruct!>> -> 5
+! CHECK: 2 ^IfStmt -> 5: 10if(i.eq.1)goto11
+! CHECK: 3 ^GotoStmt! -> 7: goto11
+