What you see happening is a result of the rules of integer promotions. Anytime a variable smaller than an int is used in an expression the value is promoted to type int.
Suppose bufi[i] contains the value 255. The hex representation of this is 0xFF. This value is then operand of the ~ operator. So the value will first be promoted to int which (assuming it is 32 bit) will have the value 0x000000FF, and applying ~ to this gives you 0xFFFFFF00. You then compare this value with buf[i] which is of type uint8_t. The value 0xFFFFFF00 is outside of this range so the comparison will always be false.
If you assign the result of the ~ back to a variable of type uint8_t, the value 0xFFFFFF00 is converted to 0x00. It is this converted value that is then compared against buf[i].
So the behavior you see is not the result of an optimization but the rules of the language. Using a temp variable as you are is one way to address this issue. You could also cast the result to uint8:
if(buf[i] != (uint8)(~bufi[i]))
Or mask out all but the lowest order byte:
if(buf[i] != (~bufi[i] & 0xff))