Don't write to _ if it's never read

This is common in code like:

```python
for _ in range(10):
    pass
```

Author: tekknolagi

library/_compiler.py

@@ -211,6 +211,21 @@ def visitBinOp(self, node: ast.BinOp) -> ast.expr:
 class PyroFlowGraph(PyFlowGraph38):
     opcode = opcodepyro.opcode
 
+    def optimizeStoreUnderscore(self):
+        for block in self.getBlocksInOrder():
+            for instr in block.getInstructions():
+                if (
+                    instr.opname == "LOAD_FAST" or instr.opname == "DELETE_FAST"
+                ) and instr.oparg == "_":
+                    return
+        # We never read from or delete _, so we can replace all stores to it
+        # with POP_TOP.
+        for block in self.getBlocksInOrder():
+            for instr in block.getInstructions():
+                if instr.opname == "STORE_FAST" and instr.oparg == "_":
+                    instr.opname = "POP_TOP"
+                    instr.oparg = None
+
     def optimizeLoadFast(self):
         blocks = self.getBlocksInOrder()
         preds = tuple(set() for i in range(self.block_count))
@@ -297,6 +312,7 @@ def process_one_block(block, modify=False):
             self.entry.insts = deletes + self.entry.insts
 
     def getCode(self):
+        self.optimizeStoreUnderscore()
         self.optimizeLoadFast()
         return super().getCode()