[LateLowerGCFrame] fix PlaceGCFrameReset for returns_twice (JuliaLang#57392)

Using the right variable here should help quite a bit with the random GC
segfaults we have seen. We already have the tests for this, but it is
quite hard to make them just complex enough to trigger reliably.

Fixes JuliaLang#57333

Author: vtjnash

src/codegen.cpp

@@ -3232,7 +3232,6 @@ static bool slot_eq(jl_value_t *e, int sl)
 // --- find volatile variables ---
 
 // assigned in a try block and used outside that try block
-
 static bool local_var_occurs(jl_value_t *e, int sl)
 {
     if (slot_eq(e, sl)) {
@@ -3272,13 +3271,13 @@ static bool have_try_block(jl_array_t *stmts)
     return 0;
 }
 
-// conservative marking of all variables potentially used after a catch block that were assigned before it
+// conservative marking of all variables potentially used after a catch block that were assigned after the try
 static void mark_volatile_vars(jl_array_t *stmts, SmallVectorImpl<jl_varinfo_t> &slots, const std::set<int> &bbstarts)
 {
     if (!have_try_block(stmts))
         return;
     size_t slength = jl_array_dim0(stmts);
-    BitVector assigned_in_block(slots.size()); // conservatively only ignore slots assigned in the same basic block
+    BitVector assigned_in_block(slots.size()); // since we don't have domtree access, conservatively only ignore slots assigned in the same basic block
     for (int j = 0; j < (int)slength; j++) {
         if (bbstarts.count(j + 1))
             assigned_in_block.reset();

src/llvm-late-gc-lowering.cpp

@@ -2291,7 +2291,7 @@ void LateLowerGCFrame::PlaceGCFrameStores(State &S, unsigned MinColorRoot,
             const LargeSparseBitVector &NowLive = S.LiveSets[*rit];
             // reset slots which are no longer alive
             for (int Idx : *LastLive) {
-                if (Idx >= PreAssignedColors && !HasBitSet(NowLive, Idx)) {
+                if (Colors[Idx] >= PreAssignedColors && !HasBitSet(NowLive, Idx)) {
                     PlaceGCFrameReset(S, Idx, MinColorRoot, Colors, GCFrame,
                       S.ReverseSafepointNumbering[*rit]);
                 }