Multi-phase drreg use hides app value from later phases

Today, DR supports multi-phase use for drreg, which means that clients can use drreg to get scratch regs in insertion phase and other phases as well.

#3823 added support to avoid spill slot conflicts in multi-phase use. However, different phases can still get the same scratch reg. In such "nested" spill regions, `drreg_get_app_value` won't work as expected in later phases, as it will give the value of the reg in the earlier phase, not the app value. This should be fixed, and documented until then.

Another related note:
Drreg support in non-insertion phases is bare bones -- e.g. it does not automatically restore app value before app read, or re-spill app value after app writes. This is usually fine because non-insertion phase is pretty limited. Today, we do that to expand scatter/gather instructions in app2app phase. But we should document this restriction about non-insertion phase use.