Remove duplicate strtoupper

Removing the duplicate strtoupper call has a meaningful impact on
performance since this method is called at least once per cell.

`Worksheet::getCells` currently calls `strtoupper` twice. `strtoupper`
is kind of expensive and this method is called at least once for every
cell in the spreadsheet.  By removing the unnecessary second call the
runtime decreases by 18% when importing a ~100K cell spreadsheet.

Closes PHPOffice#825

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 - Support COUNTIFS multiple arguments - [#830](https://github.com/PHPOffice/PhpSpreadsheet/pull/830)
 - Change `libxml_disable_entity_loader()` as shortly as possible - [#819](https://github.com/PHPOffice/PhpSpreadsheet/pull/819)
 - Improved memory usage and performance when loading large spreadsheets - [#822](https://github.com/PHPOffice/PhpSpreadsheet/pull/822)
+- Improved performance when loading large spreadsheets - [#825](https://github.com/PHPOffice/PhpSpreadsheet/pull/825)
 
 ## [1.5.2] - 2018-11-25
 

src/PhpSpreadsheet/Worksheet/Worksheet.php

@@ -1191,9 +1191,12 @@ public function setCellValueExplicitByColumnAndRow($columnIndex, $row, $value, $
      */
     public function getCell($pCoordinate, $createIfNotExists = true)
     {
+        // Uppercase coordinate
+        $pCoordinateUpper = strtoupper($pCoordinate);
+
         // Check cell collection
-        if ($this->cellCollection->has(strtoupper($pCoordinate))) {
-            return $this->cellCollection->get($pCoordinate);
+        if ($this->cellCollection->has($pCoordinateUpper)) {
+            return $this->cellCollection->get($pCoordinateUpper);
         }
 
         // Worksheet reference?
@@ -1214,17 +1217,14 @@ public function getCell($pCoordinate, $createIfNotExists = true)
             }
         }
 
-        // Uppercase coordinate
-        $pCoordinate = strtoupper($pCoordinate);
-
         if (Coordinate::coordinateIsRange($pCoordinate)) {
             throw new Exception('Cell coordinate can not be a range of cells.');
         } elseif (strpos($pCoordinate, '$') !== false) {
             throw new Exception('Cell coordinate must not be absolute.');
         }
 
         // Create new cell object, if required
-        return $createIfNotExists ? $this->createNewCell($pCoordinate) : null;
+        return $createIfNotExists ? $this->createNewCell($pCoordinateUpper) : null;
     }
 
     /**