Fix a use-after-free in MandatoryInlining

In recordDeadFunction, we look at operands of an instruction to be deleted,
and add back the defining instruction of the operands to the worklist.
This works in general when we are deleting dead instructions
recursively.
But we also consider, an instruction with only debug uses as dead. So
when we are deleting a debug instruction, we may have already deleted
its operand's defining instruction. So it would be incorrect to add it
to the worklist.

Author: meg-gupta

lib/SILOptimizer/Mandatory/MandatoryInlining.cpp

@@ -473,6 +473,19 @@ class ClosureCleanup {
   /// This regular instruction deletion callback checks for any function-type
   /// values that may be unused after deleting the given instruction.
   void recordDeadFunction(SILInstruction *deletedInst) {
+    // If it is a debug instruction, return.
+    // In this function, we look at operands of an instruction to be
+    // deleted, and add back the defining instruction of the operands to the
+    // worklist if it has a function type. This works in general when we are
+    // deleting dead instructions recursively.
+    // But we also consider, an instruction with only debug uses as dead.
+    // And with eraseFromParentWithDebugInsts, we will be deleting a dead
+    // instruction with its debug instructions. So when we are deleting a debug
+    // instruction, we may have already deleted its operand's defining
+    // instruction. So it would be incorrect to add back its operand's defining
+    // instruction.
+    if (deletedInst->isDebugInstruction())
+      return;
     // If the deleted instruction was already recorded as a function producer,
     // delete it from the map and record its operands instead.
     deadFunctionVals.erase(deletedInst);