drreg ever_spilled state never reset on new block

Xref #3821
Xref https://github.com/DynamoRIO/dynamorio/pull/3822#discussion_r322862007

drreg's per-reg ever_spilled state does not seem to be reset, ever. It seems like it should be set to false at the start of a new block, in drreg_event_bb_analysis next to where app_uses=0 or sthg like that. It also seems like failing to reset it might affect restores: though maybe not in common use patterns since there's other state that controls restores, so maybe there are no consequence to this. This issue covers investigating further and fixing.