Added SSE2 variants for Average2/3/4
Added SSE2 variants for Average2/3/4

The predictors based on Average2 are tad slower.

Following is the performance data for these predictors normalized to
number of instruction cycles (as per valgrind) per operation:
- Predictor6 & Predictor7 now takes 15 instruction cycles compared to 11
  instruction cycles for the C version.
- Predictor8 & Predictor9 now takes 15 instruction cycles compared to 12
  instruction cycles for the C version.

The predictors based on Average4 is faster and Average3 is tad slower:
- Predictor10 (Average4) now takes 23 instruction cycles compared to 25
  instruction cycles for the C version.
- Predictor5 (Average3) now takes 20 instruction cycles compared to 18
  instruction cycles for the C version.

Maybe SSE2 version of Average2 can be improved further. Otherwise, we can
remove the SSE2 version and always fallback to the C version.

Change-Id: I388b2871919985bc28faaad37c1d4beeb20ba029

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