tree-optimization/99856 - fix testcase

When making the testcase use aligned accesses I botched up the
copy&paste.  Fixed.

	PR tree-optimization/99856
	* gcc.dg/vect/pr99856.c: Fix copy&paste errors.

gcc/testsuite/gcc.dg/vect/pr99856.c

@@ -17,8 +17,8 @@ opSourceOver_premul(uint8_t* restrict Rrgba,
                     const uint8_t* restrict Drgba, int len)
 {
   Rrgba = __builtin_assume_aligned (Rrgba, __BIGGEST_ALIGNMENT__);
-  Srgba = __builtin_assume_aligned (Rrgba, __BIGGEST_ALIGNMENT__);
-  Drgba = __builtin_assume_aligned (Rrgba, __BIGGEST_ALIGNMENT__);
+  Srgba = __builtin_assume_aligned (Srgba, __BIGGEST_ALIGNMENT__);
+  Drgba = __builtin_assume_aligned (Drgba, __BIGGEST_ALIGNMENT__);
   int i = 0;
   for (; i < len*4; i += 4)
     {