[Pipeliner] Fix an assertion caused by iterator invalidation.

llvm/lib/CodeGen/MachinePipeliner.cpp

for (SUnit &SU : DAG->SUnits) {
     }
 
     if (PHISUs.size() == 0 || SrcSUs.size() == 0)
       continue;
 
     // Find the USEs of PHI. If the use is a PHI or REG_SEQUENCE, push back this
     // SUnit to the container.
     SmallVector<SUnit *, 8> UseSUs;
-    for (auto I = PHISUs.begin(); I != PHISUs.end(); ++I) {
-      for (auto &Dep : (*I)->Succs) {
+    // Do not use iterator based loop here as we are updating the container.
+    for (size_t Index = 0; Index < PHISUs.size(); ++Index) {
+      for (auto &Dep : PHISUs[Index]->Succs) {
         if (Dep.getKind() != SDep::Data)
           continue;
 
         SUnit *TmpSU = Dep.getSUnit();
         MachineInstr *TmpMI = TmpSU->getInstr();
         if (TmpMI->isPHI() || TmpMI->isRegSequence()) {
           PHISUs.push_back(TmpSU);
           continue;

llvm/test/CodeGen/Hexagon/swp-crash-iter.ll

+; REQUIRES: asserts
+; RUN: llc -march=hexagon -enable-pipeliner -o /dev/null < %s
+; Test that we do not crash when running CopyToPhi DAG mutation due to
+; iterator invalidation.
+
+declare i64 @llvm.hexagon.M2.cmacsc.s0(i64, i32, i32) #0
+define dso_local void @lte_LL1_srch_pss_proc_seq_cmplx_mult_no_normalize() local_unnamed_addr #1 {
+entry:
+  br label %for.body
+for.body:                                         ; preds = %for.body, %entry
+  %loop_count.0420 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %tmp_rslt4.sroa.0.0415 = phi i64 [ 0, %entry ], [ %phitmp395, %for.body ]
+  %tmp_rslt4.sroa.14.0414 = phi i64 [ 0, %entry ], [ %phitmp394, %for.body ]
+  %tmp_rslt3.sroa.12.0412 = phi i64 [ 0, %entry ], [ %phitmp391, %for.body ]
+  %tmp_rslt3.sroa.0.0.insert.insert = or i64 0, %tmp_rslt3.sroa.12.0412
+  %0 = tail call i64 @llvm.hexagon.M2.cmacsc.s0(i64 %tmp_rslt3.sroa.0.0.insert.insert, i32 undef, i32 undef)
+  %tmp_rslt4.sroa.0.0.insert.insert = or i64 %tmp_rslt4.sroa.0.0415, %tmp_rslt4.sroa.14.0414
+  %1 = tail call i64 @llvm.hexagon.M2.cmacsc.s0(i64 %tmp_rslt4.sroa.0.0.insert.insert, i32 undef, i32 undef)
+  %inc = add nuw nsw i32 %loop_count.0420, 1
+  %phitmp391 = and i64 %0, -4294967296
+  %phitmp394 = and i64 %1, -4294967296
+  %phitmp395 = and i64 %1, 4294967295
+  %exitcond = icmp eq i32 %inc, 63
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  %2 = tail call i64 @llvm.hexagon.M2.cmacsc.s0(i64 %0, i32 undef, i32 undef)
+  ret void
+}
+
+attributes #0 = { nounwind readnone }
+attributes #1 = { "target-features"="+v66,-long-calls,-small-data" }