planner: fix panic of merge join for tables with redundant indexes (#15840)#15919

Merged

zz-jason merged 4 commits intopingcap:release-3.0from sre-bot:release-3.0-10de222741de

Apr 1, 2020

+13-1

Commits on Mar 27, 2020

resolve conflicts
eurekaka
committed

Commits on Mar 31, 2020

cherry pick pingcap#15840 to release-3.0

eurekaka
authored and
sre-bot
committed

Commits on Apr 1, 2020