Do not drop negation if resulting type isn't boolean

idea/testData/intentions/convertNegatedExpressionWithDemorgansLaw/doubleNegation.kt

@@ -1,4 +1,3 @@
-// SKIP_ERRORS_AFTER
 operator fun String.not(): Boolean {
     return length == 0
 }

idea/testData/intentions/convertNegatedExpressionWithDemorgansLaw/doubleNegation.kt.after

@@ -1,8 +1,7 @@
-// SKIP_ERRORS_AFTER
 operator fun String.not(): Boolean {
     return length == 0
 }
 
 fun foo(a: Boolean, b: Boolean) : Boolean {
-    return "" && !b
+    return !!"" && !b
 }