scx: Branchless implementation of highest_bit

Origin implementation of function highest_bit utilize the function of
"fls()" to calculate the most significant bit of the input parameter
"flags". Normally we can return the mask with "1 << (fls(flags) - 1)",
but "fls(flags)" will return 0 if the value of "flags" is 0, which will
cause the evaluation become "1 << (-1)" and it's illegal. So we use a
branch to determine whether the return value of "fls(flags)" is 0.

We can remove the use of branch first left shift "fls(flags)" number of
bits and then right shift 1 bit. When the value of "fls(flags)" is 0
then this evaluation will simply become 0 without any error. As more
values other than 0, evaluation is the same as it did for
"1 << (fls(flags) - 1)".

This implementation can prevent any possible branch prediction fault and
pure shift operations are cheaper than branch operation for if-else
statements.

kernel/sched/ext.c

@@ -1017,7 +1017,7 @@ static u32 higher_bits(u32 flags)
 static u32 highest_bit(u32 flags)
 {
 	int bit = fls(flags);
-	return bit ? 1 << (bit - 1) : 0;
+	return ((u64) 1 << bit) >> 1;
 }
 
 /*