Vulkan: Move pruneOrphanedBufferBlocks to last

RendererVk::cleanupGarbage() should call pruneOrphanedBufferBlocks()
after clean up mSuballocationGarbage, so that we will be able to find
the orphaned buffer block being empty if mSuballocationGarbage actually
made it empty.

Bug: chromium:1318110
Change-Id: I0cc6e45c192f406e7bbb15c7427d30e06208dd06
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3611648
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>

src/libANGLE/renderer/vulkan/RendererVk.cpp

@@ -3831,11 +3831,6 @@ angle::Result RendererVk::cleanupGarbage(Serial lastCompletedQueueSerial)
 {
     std::lock_guard<std::mutex> lock(mGarbageMutex);
 
-    if (!mOrphanedBufferBlocks.empty())
-    {
-        pruneOrphanedBufferBlocks();
-    }
-
     // Clean up general garbages
     while (!mSharedGarbage.empty())
     {
@@ -3858,6 +3853,13 @@ angle::Result RendererVk::cleanupGarbage(Serial lastCompletedQueueSerial)
         mSuballocationGarbage.pop();
     }
 
+    // Note: do this after clean up mSuballocationGarbage so that we will have more chances to find
+    // orphaned blocks being empty.
+    if (!mOrphanedBufferBlocks.empty())
+    {
+        pruneOrphanedBufferBlocks();
+    }
+
     return angle::Result::Continue;
 }