Fixes a drreg bug in drreg_spill_aflags() which set xax's ever_spilled flag to true even if it wasn't spilled because it was dead. This caused requests to get the app value of xax to return success and to move aflags out of xax and into a slot, but without restoring the app value. The fixed behavior is to fail up front if xax is dead and so leave aflags in xax.
Updates the drreg tests that rely on getting the xax value moving aflags into a slot to use a live xax rather than a dead xax.
This was discovered while adding PR #5164 for #4128 (closed).
Issue: #4128 (closed)