Wrong inference for shift operators

[lnicola]

    let _x = 1u32 << 5u8;

_x gets inferred as u8. Probably caused by https://github.com/rust-analyzer/rust-analyzer/pull/451/files#diff-bd22dd45f1f2ee115460eaa92ea6b1c8R555-R556.

[detrumi]

Currently binary_op_return_ty always uses the right-hand type as result type, but this is wrong for <<, which uses the left-hand type.

More generally, we should resolve the associated output type like described here.