Optimize multiplications/divisions by 2 using bit shifts in hot paths

Hi Hackers,

This patch uses left/right shift operators to optimize *2 and /2 operations
in some functions that are in critical paths.

For unsigned int, *2 and /2 exactly equal to <<1 and >>1.

For signed int:
* /2 exactly equals to >>1 when a value is non-negative, this
patch ensures only changes non-negative variables
* *2 might be different from <<1 if a value is close to MAX_INT, however,
in that case x*2 will result in a undefined result, thus existing code
should have ensured *2 should not exceed MAX_INT.

"make check" passes with this patch.

Best regards,
Chao Li (Evan)
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HighGo Software Co., Ltd.
https://www.highgo.com/