remove the special casing for res->first in VP8SetResidualCoeffs

if res->first = 1, coeffs[0]=0 because of quant.c:749 and line
added at quant.c:744
So, no need for the extra case.
Going forward, TrellisQuantizeBlock() should also be calling
a variant of VP8SetResidualCoeffs() to set the 'last' field.

also: fixes a warning for win64
    + slight speed-up

Change-Id: Ib24b611f7396d24aeb5b56dc74d5c39160f048f0

src/dsp/enc_sse2.c

@@ -946,14 +946,14 @@ void VP8SetResidualCoeffsSSE2(const int16_t* const coeffs,
   // Get the comparison results as a bitmask, consisting of two times 16 bits:
   // two identical bits for each result. Concatenate both bitmasks to get a
   // single 32 bit value. Negate the mask to get the position of entries that
-  // are not equal to zero. Finally, mask out least significant bits according
-  // to res->first.
+  // are not equal to zero. We don't need to mask out least significant bits
+  // according to res->first, since coeffs[0] is 0 if res->first > 0
   const uint32_t mask =
-      ~(((uint32_t)_mm_movemask_epi8(m1) << 16) | _mm_movemask_epi8(m0)) &
-      -(1U << (res->first << 1));
+      ~(((uint32_t)_mm_movemask_epi8(m1) << 16) | _mm_movemask_epi8(m0));
   // The position of the most significant non-zero bit indicates the position of
   // the last non-zero value. Divide the result by two because __movemask_epi8
   // operates on 8 bit values instead of 16 bit values.
+  assert(res->first == 0 || coeffs[0] == 0);
   res->last = mask ? (BitsLog2Floor(mask) >> 1) : -1;
   res->coeffs = coeffs;
 }