Adds optimizations for by-ref types to:
  - .struct_field_val
  - .slice_elem_val
  - .ptr_elem_val

I would have expected LLVM to be able to optimize away these
temporaries since we don't leak pointers to them and they are fed
straight from def to use, but empirically it does not.

Resolves https://github.com/ziglang/zig/issues/12713
Resolves https://github.com/ziglang/zig/issues/12638