[mypyc] Fix invalid unlikely() in certain rare branches (#11939)

If we switch true/false branches, we also need to switch between
likely/unlikely.

This has an impact at least on accessing module-level final attributes
with non-constant initializers. I couldn't see a significant
difference in benchmark results compared to master, but this seems to 
help together with some other work-in-progress improvements I have 
been working on.

mypyc/codegen/emitfunc.py

@@ -122,10 +122,12 @@ def visit_goto(self, op: Goto) -> None:
     def visit_branch(self, op: Branch) -> None:
         true, false = op.true, op.false
         negated = op.negated
+        negated_rare = False
         if true is self.next_block and op.traceback_entry is None:
             # Switch true/false since it avoids an else block.
             true, false = false, true
             negated = not negated
+            negated_rare = True
 
         neg = '!' if negated else ''
         cond = ''
@@ -150,7 +152,10 @@ def visit_branch(self, op: Branch) -> None:
 
         # For error checks, tell the compiler the branch is unlikely
         if op.traceback_entry is not None or op.rare:
-            cond = 'unlikely({})'.format(cond)
+            if not negated_rare:
+                cond = 'unlikely({})'.format(cond)
+            else:
+                cond = 'likely({})'.format(cond)
 
         if false is self.next_block:
             if op.traceback_entry is None:

mypyc/test/test_emitfunc.py

@@ -187,6 +187,29 @@ def test_branch_is_error_next_block(self) -> None:
                          """if (cpy_r_b == 2) goto CPyL9;""",
                          next_block=next_block)
 
+    def test_branch_rare(self) -> None:
+        self.assert_emit(Branch(self.b, BasicBlock(8), BasicBlock(9), Branch.BOOL, rare=True),
+                         """if (unlikely(cpy_r_b)) {
+                                goto CPyL8;
+                            } else
+                                goto CPyL9;
+                         """)
+        next_block = BasicBlock(9)
+        self.assert_emit(Branch(self.b, BasicBlock(8), next_block, Branch.BOOL, rare=True),
+                         """if (unlikely(cpy_r_b)) goto CPyL8;""",
+                         next_block=next_block)
+        next_block = BasicBlock(8)
+        b = Branch(self.b, next_block, BasicBlock(9), Branch.BOOL, rare=True)
+        self.assert_emit(b,
+                         """if (likely(!cpy_r_b)) goto CPyL9;""",
+                         next_block=next_block)
+        next_block = BasicBlock(8)
+        b = Branch(self.b, next_block, BasicBlock(9), Branch.BOOL, rare=True)
+        b.negated = True
+        self.assert_emit(b,
+                         """if (likely(cpy_r_b)) goto CPyL9;""",
+                         next_block=next_block)
+
     def test_call(self) -> None:
         decl = FuncDecl('myfn', None, 'mod',
                         FuncSignature([RuntimeArg('m', int_rprimitive)], int_rprimitive))