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/enc/cost.c

@@ -562,7 +562,8 @@ static void SetResidualCoeffs(const int16_t* const coeffs,
                               VP8Residual* const res) {
   int n;
   res->last = -1;
-  for (n = 15; n >= res->first; --n) {
+  assert(res->first == 0 || coeffs[0] == 0);
+  for (n = 15; n >= 0; --n) {
     if (coeffs[n]) {
       res->last = n;
       break;