transform (MakeGCStackSlots): do not move the stack chain pop earlier

Previously, the MakeGCStackSlots pass would attempt to pop the stack chain before a tail call.
This resulted in use-after-free bugs when the tail call allocated memory and used a value allocated by its caller.
Instead of trying to move the stack chain pop, remove the tail flag from the call.

transform/gc.go

@@ -271,37 +271,17 @@ func MakeGCStackSlots(mod llvm.Module) bool {
 		// Make sure this stack object is popped from the linked list of stack
 		// objects at return.
 		for _, ret := range returns {
-			inst := ret
-			// Try to do the popping of the stack object earlier, by inserting
-			// it not right before the return instruction but moving the insert
-			// position up.
-			// This is necessary so that the GC stack slot pass doesn't
-			// interfere with tail calls (in particular, musttail calls).
-			for {
-				prevInst := llvm.PrevInstruction(inst)
-				if prevInst == parent {
-					break
-				}
-				if _, ok := pointerStores[prevInst]; ok {
-					// Pop the stack object after the last store instruction.
-					// This can probably be made more efficient: storing to the
-					// stack chain object and then immediately popping isn't
-					// useful.
-					break
-				}
-				if prevInst.IsNil() {
-					// Start of basic block. Pop the stack object here.
-					break
-				}
-				if !prevInst.IsAPHINode().IsNil() {
-					// Do not insert before a PHI node. PHI nodes must be
-					// grouped at the beginning of a basic block before any
-					// other instruction.
-					break
-				}
-				inst = prevInst
+			// Check for any tail calls at this return.
+			prev := llvm.PrevInstruction(ret)
+			if !prev.IsNil() && !prev.IsABitCastInst().IsNil() {
+				// A bitcast can appear before a tail call, so skip backwards more.
+				prev = llvm.PrevInstruction(prev)
 			}
-			builder.SetInsertPointBefore(inst)
+			if !prev.IsNil() && !prev.IsACallInst().IsNil() {
+				// This is no longer a tail call.
+				prev.SetTailCall(false)
+			}
+			builder.SetInsertPointBefore(ret)
 			builder.CreateStore(parent, stackChainStart)
 		}
 	}