Auto merge of #74821 - oli-obk:const_eval_read_uninit_fast_path, r=wesleywiser

Auto merge of #74821 - oli-obk:const_eval_read_uninit_fast_path, r=wesleywiser

Check whether locals are too large instead of whether accesses into them are too large

Essentially this stops const prop from attempting to optimize

```rust
let mut x = [0_u8; 5000];
x[42] = 3;
```

I don't expect this to be a perf improvement without #73656 (which is also where the lack of this PR will be a perf regression).

r? @wesleywiser