Rollup merge of #107687 - cjgillot:sroa-2, r=oli-obk

Adapt SROA MIR opt for aggregated MIR

The pass was broken by https://github.com/rust-lang/rust/pull/107267.

This PR extends it to replace:
```
x = Struct { 0: a, 1: b }
y = move? x
```

by assignment between locals
```
x_0 = a
x_1 = b
y_0 = move? x_0
y_1 = move? x_1
```

The improved pass runs to fixpoint, so we can flatten nested field accesses.