Clean up / speed up various SIMD functions

• In InterPrediction::rightShiftMSB, use existing floorLog2 function

• In addAvg_SSE, avoid unnecessary use of 32-bit path

• In copyBufferSimd, reduce number of loops

• In paddingSimd, take advantage of padding extent being either 1 or 2

• In addBIOAvg4_SSE, reduce number of operations and avoid nasty Xmm register -> memory -> integer register path

Overall, a decoder runtime reduction of about 3% is expected