Rollup merge of #61404 - RalfJung:miri-unsize, r=oli-obk
Rollup merge of #61404 - RalfJung:miri-unsize, r=oli-obk

miri unsizing: fix projecting into a field of an operand

I don't know why this open-coded an operand field projection. Probably this code predates one or more of my refactorings.

Fixes https://github.com/rust-lang/miri/issues/754

r? @oli-obk

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust