Write guards on auto-sliced index expressions probably do not work

[nikomatsakis]

I am pretty sure that the logic is slightly wrong if you have a program like this:

let y = @mut [3];
let x: ~[@mut [int]] = ~[y];
take_imm_slice(x[0]);

fn take_imm_slice<'a>(v: &[int]) { ... }

note here that the result of x[0] is being auto-sliced. The fix is to use an "derefs" count of 1 for auto-slicing, I think, but I did not want to disturb the currently working code with this change until I can land the current version.

cc #5074

[nikomatsakis]

fixed in PR #6373