KT-4553 Inspection to detect if's that are foldable as "?."

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