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CodeGen: rework Aggregate implemention for rvalue_creates_operand cases
A non-trivial refactor pulled out from rust-lang/rust#138759
r? workingjubilee
The previous implementation I'd written here based on `index_by_increasing_offset` is complicated to follow and difficult to extend to non-structs.
This changes the implementation, without actually changing any codegen (thus no test changes either), to be more like the existing `extract_field` (<2b0274c71d/compiler/rustc_codegen_ssa/src/mir/operand.rs (L345-L425)>) in that it allows setting a particular field directly.
Notably I've found this one much easier to get right, in particular because having the `OperandRef<Result<V, Scalar>>` gives a really useful thing to include in ICE messages if something did happen to go wrong.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.