dec_sse2: harmonize function suffixes
dec_sse2: harmonize function suffixes

BUG=webp:355

Change-Id: Ic0390a4a24a5d8caff5b8af9fc9d59769ec533b1

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