gh-116088: Insert bottom checks after all sym_set_...() calls (#116089)

This changes the `sym_set_...()` functions to return a `bool` which is `false`
when the symbol is `bottom` after the operation.

All calls to such functions now check this result and go to `hit_bottom`,
a special error label that prints a different message and then reports
that it wasn't able to optimize the trace. No executor will be produced
in this case.

Tools/cases_generator/optimizer_generator.py

@@ -4,16 +4,12 @@ Writes the cases to optimizer_cases.c.h, which is #included in Python/optimizer_
 """
 
 import argparse
-import os.path
-import sys
 
 from analyzer import (
     Analysis,
     Instruction,
     Uop,
-    Part,
     analyze_files,
-    Skip,
     StackItem,
     analysis_error,
 )