air, codegen: copy elision

Currently, air supports any type (including aggregates) as temporaries. Codegen lowers all aggregates to the stack, which results in some redundant intermediates being created.

For example, the code `let mut slice = array[start..end]` creates two allocas: the first one is the temporary to store the slicing result of array[start..end], and the second is the mutable stack slot for the `slice` variable. The backend then generates a copy (unrolled gep + load + store for slices, memcpy for arrays) between the two allocas. This problem doesn't currently happen when `slice` is constant, because no stack slot is created. However, for mut slices, we should find a way to elide the copy, and intialize the rvalue directly into the mut variable.