[mypyc] feat: support constant folding in transform_assignment_expr

We can't yet fold the expression itself, but we can fold its value expr

This PR implements folding for the value expr

Author: BobTheBuidler

mypyc/irbuild/expression.py

@@ -1097,7 +1097,7 @@ def transform_generator_expr(builder: IRBuilder, o: GeneratorExpr) -> Value:
 
 
 def transform_assignment_expr(builder: IRBuilder, o: AssignmentExpr) -> Value:
-    value = builder.accept(o.value)
+    value = try_constant_fold(builder, o.value) or builder.accept(o.value)
     target = builder.get_assignment_target(o.target)
     builder.assign(target, value, o.line)
     return value