[x265] Negative integer shifts

[Andrey Semashev]

Hi,

While compiling 2.4 I'm seeind lots of warnings like this:

.../source/common/ipfilter.cpp: In instantiation of ‘void 
{anonymous}::interp_horiz_ps_c(const pixel*, intptr_t, int16_t*, 
intptr_t, int, int) [with int N = 8; int width = 4; int height = 4; 
pixel = unsigned char; intptr_t = long int; int16_t = short int]’:
.../source/common/ipfilter.cpp:417:5:   required from here
.../source/common/ipfilter.cpp:126:36: warning: left shift of negative 
value [-Wshift-negative-value]
      int offset = -IF_INTERNAL_OFFS << shift;
                   ~~~~~~~~~~~~~~~~~~^~~~~~~~

Left-shifting negative signed intehers is undefined behavior in C++. 
I've attached a patch that resolves the warnings. The patch assumes 2's 
complement signed integers and that the shift does not introduce an 
arithmetic overflow.
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