- In SSSE3 optimization, 16-bit addition and subtraction would
  overflow when input coefficient is 16-bit signed extreme values.
- Function-level speed becomes slower (unit ms):
  idct8x8_64: 284 -> 294
  idct8x8_12: 145 -> 158.

BUG=webm:1332

Change-Id: I1e4bf9d30a6d4112b8cac5823729565bf145e40b