Rollup merge of #142383 - scottmcm:operandref-builder, r=workingjubilee

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` (<https://github.com/rust-lang/rust/blob/2b0274c71dba0e24370ebf65593da450e2e91868/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.

Author: Kobzol