[mypyc] Mark magic-overlapping error handler branches as rare (#13525)

This might slightly improve performance when using native ints,
and it's consistent with how other error handler branches are
treated as rare.

Now the generated error handling code when calling a function
that returns a native int looks something like this (the
`unlikely(...)` part is new):

```
    cpy_r_r0 = CPyDef_f();
    cpy_r_r1 = cpy_r_r0 == -113;
    if (unlikely(cpy_r_r1)) goto CPyL2;
```

Author: JukkaL

mypyc/transform/exceptions.py

@@ -103,7 +103,11 @@ def split_blocks_at_errors(
                     new_block2 = BasicBlock()
                     new_blocks.append(new_block2)
                     branch = Branch(
-                        comp, true_label=new_block2, false_label=new_block, op=Branch.BOOL
+                        comp,
+                        true_label=new_block2,
+                        false_label=new_block,
+                        op=Branch.BOOL,
+                        rare=True,
                     )
                     cur_block.ops.append(branch)
                     cur_block = new_block2