Use TCU_CHECK instead of DE_ASSERT

DE_ASSERT is optimized away in release build. In this case it resulted
in the 'modifyXor' function not being called, causing the reference
memory to go out of sync with the actual memory.
diff --git a/external/vulkancts/modules/vulkan/memory/vktMemoryMappingTests.cpp b/external/vulkancts/modules/vulkan/memory/vktMemoryMappingTests.cpp
index 60d2bec..475433f 100644
--- a/external/vulkancts/modules/vulkan/memory/vktMemoryMappingTests.cpp
+++ b/external/vulkancts/modules/vulkan/memory/vktMemoryMappingTests.cpp
@@ -525,7 +525,7 @@
 		const deUint8	mask	= rng.getUint8();
 
 		((deUint8*)m_ptr)[pos]	= val ^ mask;
-		DE_ASSERT(m_reference.modifyXor((size_t)(m_range.offset + pos), val, mask));
+		TCU_CHECK(m_reference.modifyXor((size_t)(m_range.offset + pos), val, mask));
 	}
 }