Remove redundant backticks: fix false positive for multiple underscores

#KT-37496 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/RemoveRedundantBackticksInspection.kt

@@ -46,9 +46,8 @@ class RemoveRedundantBackticksInspection : AbstractKotlinInspection() {
         }
     }
 
-    private fun isKeyword(text: String): Boolean {
-        return text == "yield" || text == "_" || (KtTokens.KEYWORDS.types + KtTokens.SOFT_KEYWORDS.types).any { it.toString() == text }
-    }
+    private fun isKeyword(text: String): Boolean =
+        text == "yield" || text.all { it == '_' } || (KtTokens.KEYWORDS.types + KtTokens.SOFT_KEYWORDS.types).any { it.toString() == text }
 
     private fun isRedundantBackticks(node: ASTNode): Boolean {
         val text = node.text

idea/testData/inspectionsLocal/removeRedundantBackticks/underscores.kt

@@ -0,0 +1,4 @@
+// PROBLEM: none
+fun test() {
+    val <caret>`__` = ""
+}

idea/testData/inspectionsLocal/removeRedundantBackticks/underscores2.kt

@@ -0,0 +1,4 @@
+// PROBLEM: none
+fun test() {
+    val <caret>`__________` = ""
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -8819,6 +8819,16 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
             runTest("idea/testData/inspectionsLocal/removeRedundantBackticks/underscore.kt");
         }
 
+        @TestMetadata("underscores.kt")
+        public void testUnderscores() throws Exception {
+            runTest("idea/testData/inspectionsLocal/removeRedundantBackticks/underscores.kt");
+        }
+
+        @TestMetadata("underscores2.kt")
+        public void testUnderscores2() throws Exception {
+            runTest("idea/testData/inspectionsLocal/removeRedundantBackticks/underscores2.kt");
+        }
+
         @TestMetadata("yield.kt")
         public void testYield() throws Exception {
             runTest("idea/testData/inspectionsLocal/removeRedundantBackticks/yield.kt");