rewrite stbi__shiftsigned to use a different, faster algorithm

to avoid probelm with clang -O2 to outputting buggy code

stb_image.h

@@ -4963,21 +4963,27 @@ static int stbi__bitcount(unsigned int a)
    return a & 0xff;
 }
 
+// extract an arbitrarily-aligned N-bit value (N=bits)
+// from v, and then make it 8-bits long and fractionally
+// extend it to full full range.
 static int stbi__shiftsigned(int v, int shift, int bits)
 {
-   int result;
-   int z=0;
-
-   if (shift < 0) v <<= -shift;
-   else v >>= shift;
-   result = v;
-
-   z = bits;
-   while (z < 8) {
-      result += v >> z;
-      z += bits;
-   }
-   return result;
+   static unsigned int mul_table[9] = {
+      0,
+      0xff/*0b11111111*/, 0x55/*0b01010101*/, 0x49/*0b01001001*/, 0x11/*0b00010001*/,
+      0x21/*0b00100001*/, 0x41/*0b01000001*/, 0x81/*0b10000001*/, 0x01/*0b00000001*/,
+   };
+   static unsigned int shift_table[9] = {
+      0, 0,0,1,0,2,4,6,0,
+   };
+   if (shift < 0)
+      v <<= -shift;
+   else
+      v >>= shift;
+   assert(v >= 0 && v < 256);
+   v >>= (8-bits);
+   assert(bits >= 0 && bits <= 8);
+   return (int) ((unsigned) v * mul_table[bits]) >> shift_table[bits];
 }
 
 typedef struct