[MINOR] Split balanced minor optimization

This commit change the table instruction in split balanced to be performed
before the cbind, and move some as.scalar a bit up.
This makes it slightly faster, but we need as a future task to improve
the split balanced therefore i have added a jira:

https://issues.apache.org/jira/browse/SYSTEMDS-3607

scripts/builtin/splitBalanced.dml

@@ -43,9 +43,9 @@ return (Matrix[Double] X_train, Matrix[Double] y_train, Matrix[Double] X_test,
         Matrix[Double] y_test) 
 {
 
+  classes = table(Y, 1)
   XY = order(target = cbind(Y, X),  by = 1, decreasing=FALSE, index.return=FALSE)
   # get the class count 
-  classes = table(XY[, 1], 1)
   split = floor(nrow(X) * splitRatio)
   start_class = 1
   train_row_s = 1 
@@ -70,13 +70,14 @@ return (Matrix[Double] X_train, Matrix[Double] y_train, Matrix[Double] X_test,
   {
     end_class = end_class + as.scalar(classes[i])
     class_t = XY[start_class:end_class, ]
+    ratio = as.scalar(classes_ratio_train[i])
 
-    train_row_e = train_row_e + as.scalar(classes_ratio_train[i]) 
+    train_row_e = train_row_e + ratio 
     test_row_e = test_row_e + as.scalar(classes_ratio_test[i]) 
 
-    outTrain[train_row_s:train_row_e, ] = class_t[1:as.scalar(classes_ratio_train[i]), ]
+    outTrain[train_row_s:train_row_e, ] = class_t[1:ratio, ]
 
-    outTest[test_row_s:test_row_e, ] = class_t[as.scalar(classes_ratio_train[i])+1:nrow(class_t), ]
+    outTest[test_row_s:test_row_e, ] = class_t[ratio+1:nrow(class_t), ]
 
     train_row_s = train_row_e + 1
     test_row_s = test_row_e + 1