[cherry-pick] fix index scan bug

[marsishandsome]

cherry-pick #995

[marsishandsome]

/run-all-tests

[codecov-io]

Codecov Report

Merging #1024 into release-2.1 will increase coverage by 0.05%.
The diff coverage is 50%.

@@                Coverage Diff                @@
##             release-2.1    #1024      +/-   ##
=================================================
+ Coverage          53.39%   53.44%   +0.05%     
  Complexity          1120     1120              
=================================================
  Files                153      153              
  Lines               7025     7020       -5     
  Branches             816      814       -2     
=================================================
+ Hits                3751     3752       +1     
+ Misses              2901     2896       -5     
+ Partials             373      372       -1

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/com/pingcap/tikv/meta/TiDAGRequest.java	41.86% <ø> (+0.18%)	61 <0> (ø)	⬇️
...cap/tikv/operation/iterator/CoprocessIterator.java	65.51% <50%> (+7.94%)	4 <0> (ø)	⬇️
.../com/pingcap/tikv/predicates/TiKVScanAnalyzer.java	57.07% <0%> (+0.19%)	31% <0%> (ø)	⬇️

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