Avoid local var entry for continuation parameter in suspend function

The primary reason is getting rid of redundant stack spilling, but also
it's not very sensible to have such entry since the parameter
is synthetic

compiler/testData/codegen/bytecodeText/coroutines/doNotReassignContinuation.kt

@@ -14,5 +14,4 @@ suspend fun suspendThere(param: Int, param2: String, param3: Long): String {
     return a + b
 }
 
-/* 2 stores happen because the continuation parameter is visible in debug and should be spilled */
-// 2 ASTORE 4
+// 0 ASTORE 4