AArch64: Add SVE2 implementation for pow2 bitmask division
In plenty of image and video processing code it's common to modify pixel values
by a widening operation and then scale them back into range by dividing by 255.
This patch adds an named function to allow us to emit an optimized sequence
when doing an unsigned division that is equivalent to:
x = y / (2 ^ (bitsize (y)/2)-1)
For SVE2 this means we generate for:
void draw_bitmap1(uint8_t* restrict pixel, uint8_t level, int n)
{
for (int i = 0; i < (n & -16); i+=1)
pixel[i] = (pixel[i] * level) / 0xff;
}