fix[lang]: fix == and != bytesM folding (#4254)

`bytesM` literals are not case-sensitive (they represent the same value
no matter if the literal is lower- or upper-case), but the folding
operation was case sensitive.

---------

Co-authored-by: Charles Cooper <cooper.charles.m@gmail.com>

tests/unit/ast/nodes/test_fold_compare.py

@@ -110,3 +110,20 @@ def test_compare_type_mismatch(op):
     old_node = vyper_ast.body[0].value
     with pytest.raises(UnfoldableNode):
         old_node.get_folded_value()
+
+
+@pytest.mark.parametrize("op", ["==", "!="])
+def test_compare_eq_bytes(get_contract, op):
+    left, right = "0xA1AAB33F", "0xa1aab33f"
+    source = f"""
+@external
+def foo(a: bytes4, b: bytes4) -> bool:
+    return a {op} b
+    """
+    contract = get_contract(source)
+
+    vyper_ast = parse_and_fold(f"{left} {op} {right}")
+    old_node = vyper_ast.body[0].value
+    new_node = old_node.get_folded_value()
+
+    assert contract.foo(left, right) == new_node.value

vyper/semantics/analysis/constant_folding.py

@@ -180,8 +180,11 @@ def visit_Compare(self, node):
             raise UnfoldableNode(
                 f"Invalid literal types for {node.op.description} comparison", node
             )
-
-        value = node.op._op(left.value, right.value)
+        lvalue, rvalue = left.value, right.value
+        if isinstance(left, vy_ast.Hex):
+            # Hex values are str, convert to be case-unsensitive.
+            lvalue, rvalue = lvalue.lower(), rvalue.lower()
+        value = node.op._op(lvalue, rvalue)
         return vy_ast.NameConstant.from_node(node, value=value)
 
     def visit_List(self, node) -> vy_ast.ExprNode: