Rollup merge of #34307 - nagisa:more-cache, r=arielb1

[MIR] Cache drops for early scope exits

Previously we would rebuild all drops on every early exit from a scope, which for code like:

```rust
match x {
    A => return 1,
    B => return 2,
    ...
    C => return 27
}
```

would produce 27 exactly same chains of drops for each return, basically a `O(n*m)` explosion. [This](https://cloud.githubusercontent.com/assets/679122/16125192/3355e32c-33fb-11e6-8564-c37cab2477a0.png) is such a case for a match on 80-variant enum with 3 droppable variables in scope.

For [`::core::iter::Iterator::partial_cmp`](https://github.com/rust-lang/rust/blob/6edea2cfda2818f0a76f4bac2d18a30feb54c137/src/libcore/iter/iterator.rs#L1909) the CFG looked like [this](https://cloud.githubusercontent.com/assets/679122/16122708/ce0024d8-33f0-11e6-93c2-e1c44b910db2.png) (after initial SimplifyCfg). With this patch the CFG looks like [this](https://cloud.githubusercontent.com/assets/679122/16122806/294fb16e-33f1-11e6-95f6-16c5438231af.png) instead.

Some numbers (overall very small wins, however neither of the crates have many cases which abuse this corner case):

|                         | old time | old rss | new time | new rss  |
|-------------------------|----------|---------|----------|----------|
| core dump               | 0.879        |   224MB     |   0.871  |  223MB   |
| core MIR passes         | 0.759        | 224MB       | 0.718    | 223MB    |
| core MIR codegen passes | 1.762        | 230MB       | 1.442    | 228MB    |
| core trans              | 3.263        | 279MB       | 3.116    | 278MB    |
| core llvm passes        | 5.611        | 263MB       | 5.565    | 263MB    |
| std dump                | 0.487        |   190MB     |   0.475  |  192MB   |
| std MIR passes          | 0.311       | 190MB       | 0.288    | 192MB    |
| std MIR codegen passes  | 0.753        | 195MB       | 0.720    | 197MB    |
| std trans               | 2.589        | 287MB       | 2.523    | 287MB    |
| std llvm passes         | 7.268        | 245MB       | 7.447    | 246MB    |

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