KT-4555 Inspection to detect if's that are foldable to elvis operator

This inspection highlights if-statements of the form:
```kotlin 
if (foo != null) foo else bar
```
and provides quickFix to:
```kotlin
foo ?: bar
```

based off of [IfThenToElvisIntention](https://github.com/jetbrains/kotlin/blob/master/idea/src/org/jetbrains/jet/plugin/intentions/branchedTransformations/intentions/IfThenToElvisIntention.kt)

All of the tests here were copied from the tests for that inspection
[testData/intentions/branched/ifThenToElvis](https://github.com/JetBrains/kotlin/tree/master/idea/testData/intentions/branched/ifThenToElvis)