Add operand size to add instructions.

In these cases it can't be guessed from the operands (at least
not necessarily), and it seems some clang versions refuse to
compiler it.
Fixes ticket #303.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/x86/snowdsp_mmx.c

@@ -675,14 +675,14 @@ static void ff_snow_vertical_compose97i_mmx(IDWTELEM *b0, IDWTELEM *b1, IDWTELEM
 
 #define snow_inner_add_yblock_sse2_end_8\
              "sal $1, %%"REG_c"              \n\t"\
-             "add $"PTR_SIZE"*2, %1          \n\t"\
+             "add"OPSIZE" $"PTR_SIZE"*2, %1  \n\t"\
              snow_inner_add_yblock_sse2_end_common1\
              "sar $1, %%"REG_c"              \n\t"\
              "sub $2, %2                     \n\t"\
              snow_inner_add_yblock_sse2_end_common2
 
 #define snow_inner_add_yblock_sse2_end_16\
-             "add $"PTR_SIZE"*1, %1          \n\t"\
+             "add"OPSIZE" $"PTR_SIZE"*1, %1  \n\t"\
              snow_inner_add_yblock_sse2_end_common1\
              "dec %2                         \n\t"\
              snow_inner_add_yblock_sse2_end_common2