[FIR] Rename FirElvisCall to FirElvisExpression

FirElvisCall is not a FirCall, so FirElvisExpression is more
  convenient name

compiler/fir/dump/src/org/jetbrains/kotlin/fir/dump/HtmlFirDump.kt

@@ -1359,10 +1359,10 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
         +"!!"
     }
 
-    private fun FlowContent.generate(elvisCall: FirElvisCall) {
-        generate(elvisCall.lhs)
+    private fun FlowContent.generate(elvisExpression: FirElvisExpression) {
+        generate(elvisExpression.lhs)
         +" ?: "
-        generate(elvisCall.rhs)
+        generate(elvisExpression.rhs)
     }
 
     @Suppress("UNUSED_PARAMETER")
@@ -1569,7 +1569,7 @@ class HtmlFirDump internal constructor(private var linkResolver: FirLinkResolver
                 is FirOperatorCall -> generate(expression)
                 is FirBinaryLogicExpression -> generate(expression)
                 is FirCheckNotNullCall -> generate(expression)
-                is FirElvisCall -> generate(expression)
+                is FirElvisExpression -> generate(expression)
                 is FirVarargArgumentsExpression -> generate(expression)
                 is FirResolvedReifiedParameterReference -> generate(expression)
                 is FirComparisonExpression -> generate(expression)