Auto merge of #35168 - scottcarr:deaggregation, r=nikomatsakis

Auto merge of #35168 - scottcarr:deaggregation, r=nikomatsakis

[MIR] Deaggregate structs to enable further optimizations

Currently, we generate MIR like:

```
tmp0 = ...;
tmp1 = ...;
tmp3 = Foo { a: ..., b: ... };
```

This PR implements "deaggregation," i.e.:

```
tmp3.0 = ...
tmp3.1 = ...
```

Currently, the code only deaggregates structs, not enums.  My understanding is that we do not have MIR to set the discriminant of an enum.