Auto merge of #105252 - bjorn3:codegen_less_pair_values, r=nagisa

Use struct types during codegen in less places

This makes it easier to use cg_ssa from a backend like Cranelift that doesn't have any struct types at all. After this PR struct types are still used for function arguments and return values. Removing those usages is harder but should still be doable.

Author: bors