Refactor UnsafeCallOnNullableType rule

[t-kameyama]

• Check KtPostfixExpression instead of KtUnaryExpression
  • We don't need to check KtPrefixExpression
• Check nullability after checking if operation is '!!'
  • We don't need to check nullability if operation is not '!!'

[codecov]

Codecov Report

Merging #3221 (1223d2f) into master (54201cd) will increase coverage by 0.02%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #3221      +/-   ##
============================================
+ Coverage     80.06%   80.09%   +0.02%     
- Complexity     2645     2651       +6     
============================================
  Files           440      440              
  Lines          8012     8013       +1     
  Branches       1535     1529       -6     
============================================
+ Hits           6415     6418       +3     
  Misses          775      775              
+ Partials        822      820       -2     

Impacted Files	Coverage Δ	Complexity Δ
...osch/detekt/rules/bugs/UnsafeCallOnNullableType.kt	77.77% <50.00%> (+5.05%)	4.00 <2.00> (ø)
...urbosch/detekt/rules/performance/ArrayPrimitive.kt	70.37% <0.00%> (-3.71%)	17.00% <0.00%> (+2.00%)	⬇️
...ab/arturbosch/detekt/rules/bugs/MissingWhenCase.kt	87.50% <0.00%> (-1.39%)	12.00% <0.00%> (+1.00%)	⬇️
...kotlin/io/gitlab/arturbosch/detekt/DetektPlugin.kt	82.53% <0.00%> (ø)	11.00% <0.00%> (ø%)
...h/detekt/core/baseline/IndentingXMLStreamWriter.kt	60.37% <0.00%> (ø)	15.00% <0.00%> (ø%)
...lab/arturbosch/detekt/rules/style/UnusedImports.kt	85.91% <0.00%> (+0.62%)	4.00% <0.00%> (ø%)
...urbosch/detekt/rules/style/UselessCallOnNotNull.kt	83.78% <0.00%> (+7.31%)	17.00% <0.00%> (+3.00%)

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