Auto merge of #118408 - RalfJung:aggregate-assign-uninit, r=saethlin

miri: add test checking that aggregate assignments reset memory to uninit

Also, `write_aggregate` is really just a helper for evaluating `Aggregate` rvalues, so it should be in `step.rs`, not `place.rs`. Also factor out `Repeat` rvalues into their own function while we are at it.

r? `@saethlin`
Fixes rust-lang/miri#3195