yuv_sse2: fix -Wconstant-conversion warning
yuv_sse2: fix -Wconstant-conversion warning

'implicit conversion from 'int' to 'short' changes value from 33050 to
-32486'

original patch:

https://codereview.chromium.org/1657313003/

Make libwebp build with -Wconstant-conversion from newer clangs.

After http://llvm.org/viewvc/llvm-project?rev=259271&view=rev, clang
points out that _mm_set1_epi16(33050) causes an overflow in the short
argument to _mm_set1_epi16().  Since there's no version that takes an
unsigned short, add an explicit cast to tell the compiler that this is
intentional.

No behavior change.

Change-Id: I6b4e3401b15cfbcc895f9e81b5c2dc59d43ffb9b

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,ttyangf/libwebp,ttyangf/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,ttyangf/libwebp,webmproject/libwebp,ttyangf/libwebp,ttyangf/libwebp