fix excute GetIndexes error with sepcial postgresql index

[xuyz]

Unsupport Indexes on Expressions

Fixed #1183

[xuyz]

@lunny
Almost all except Indexes on Expressions,

[codecov-io]

Codecov Report

Merging #1215 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1215      +/-   ##
==========================================
+ Coverage   57.37%   57.41%   +0.04%     
==========================================
  Files          43       43              
  Lines        7788     7793       +5     
==========================================
+ Hits         4468     4474       +6     
  Misses       2769     2769              
+ Partials      551      550       -1

Impacted Files	Coverage Δ
dialect_postgres.go	76.11% <100%> (+0.38%)	⬆️
xorm.go	69.69% <0%> (+1.51%)	⬆️

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[lunny]

@xuyz thanks!