further refine the COPY_PATTERN optim for DecodeAlpha

* use functions instead of MACRO
* adjust var's name

Overall, same speed, with more readible code

Change-Id: I2c3f2b12f8df15b785fad5a9c56316e954ae0c53
Merge "further refine the COPY_PATTERN optim for DecodeAlpha"
MIPS: dspr2: add some specific mips code to commit I2c3f2b12f8df15b785fad5a9c56316e954ae0c53

added some C-code tuning also

Change-Id: I67ce70a063ef6b5821b9158a4defd6987eccbb9a