Rename continuation fields according the convention and count them in IR

compiler/testData/codegen/bytecodeText/coroutines/cleanup/twoRefs.kt

@@ -0,0 +1,9 @@
+suspend fun blackhole(a: Any?) {}
+
+suspend fun cleanUpExample(a: String, b: String) {
+    blackhole(a) // 1
+    blackhole(b) // 2
+}
+
+// 3 ACONST_NULL
+// 2 PUTFIELD .*L\$0 : .*;

compiler/testData/codegen/bytecodeText/coroutines/cleanup/unusedParamNotSpill.kt

@@ -1,5 +1,5 @@
 val f: suspend (Int) -> Unit = { unused ->
 }
 
-// 0 GETFIELD p\$0
-// 0 PUTFIELD p\$0
+// 0 GETFIELD .*I\$0
+// 0 PUTFIELD .*I\$0