Don't optimize LOAD_FAST instructions whose local is killed by DEL_FAST

In certain cases it's possible for locals loaded by `LOAD_FAST` instructions
to be on the stack when the local is killed by `DEL_FAST`. These `LOAD_FAST`
instructions should not be optimized into `LOAD_FAST_BORROW` as the strong
reference in the frame is killed while there is still a reference on the stack.

Author: mpage

Lib/test/test_peepholer.py

@@ -2472,6 +2472,13 @@ def test_unoptimized_if_support_killed(self):
         ]
         self.check(insts, insts)
 
+        insts = [
+            ("LOAD_FAST", 0, 1),
+            ("DELETE_FAST", 0, 2),
+            ("POP_TOP", None, 3),
+        ]
+        self.check(insts, insts)
+
     def test_unoptimized_if_aliased(self):
         insts = [
             ("LOAD_FAST", 0, 1),

Python/flowgraph.c

@@ -2795,6 +2795,11 @@ optimize_load_fast(cfg_builder *g)
             assert(opcode != EXTENDED_ARG);
             switch (opcode) {
                 // Opcodes that load and store locals
+                case DELETE_FAST: {
+                    kill_local(instr_flags, &refs, oparg);
+                    break;
+                }
+
                 case LOAD_FAST: {
                     PUSH_REF(i, oparg);
                     break;