[ICE] crash on &= for struct

[jyn514]

Expected behavior
The parser should give an error.

Actual Behavior
RCC crashes during codegen.

Code

struct S {
    int x, y, z;
};

int main() {
    struct S s;
    struct S ss;
    s&= ss;
}
Message:  internal error: entered unreachable code: struct should not have a valid complex assignment

Backtrace

 5: rcc::ir::expr::<impl rcc::ir::Compiler>::assignment::ha78a694033530e5c
    at src/ir/expr.rs:501
 6: rcc::ir::expr::<impl rcc::ir::Compiler>::compile_expr::h5a9edf87abd8a6c3
    at src/ir/expr.rs:102
 7: rcc::ir::stmt::<impl rcc::ir::Compiler>::compile_stmt::h7778ce4d5aea5687
    at src/ir/stmt.rs:37
 8: rcc::ir::stmt::<impl rcc::ir::Compiler>::compile_all::h3fb60796f67c8f57
    at src/ir/stmt.rs:15
 9: rcc::ir::Compiler::compile_func::h9a997ea2c8bdcd6f
    at src/ir/mod.rs:322
10: rcc::ir::compile::haeb8251a574ff20c
    at src/ir/mod.rs:78
11: rcc::compile::h87633814d689a39f
    at src/lib.rs:98
12: rcc::real_main::h1dd2cb989e6fa0d8
    at src/main.rs:101
13: rcc::main::h396b827397690aa0
    at src/main.rs:169

Waiting on #151, similar to #214.