combine uncompressed

Author: Baunsgaard

src/main/java/org/apache/sysds/runtime/compress/lib/CLALibCombineGroups.java

@@ -214,10 +214,12 @@ public static AColGroup combine(AColGroup a, AColGroup b, int nRow) {
 		IColIndex combinedColumns = ColIndexFactory.combine(a, b);
 
 		// try to recompress a and b if uncompressed
-		if(a instanceof ColGroupUncompressed)
+		if( (a instanceof ColGroupUncompressed) && (b instanceof ColGroupUncompressed)){
+			// do not try to compress if both are uncompressed
+		}
+		else if(a instanceof ColGroupUncompressed)
 			a = a.recompress();
-
-		if(b instanceof ColGroupUncompressed)
+		else if(b instanceof ColGroupUncompressed)
 			b = b.recompress();
 
 		long maxEst = (long) a.getNumValues() * b.getNumValues();