perf: eliminate bounds check in PrefixSet::contains while loop

[gancerlory]

Add an explicit bounds check before the while loop in PrefixSet::contains to help the compiler eliminate redundant per-iteration bounds checks.

Closes #19189

[mattsse]

unsure if this is what you had in mind @malik672 ?

cc @DaniPopes

[malik672]

unsure if this is what you had in mind @malik672 ?

cc @DaniPopes

Nope, something like this: #19189 (comment)

[gancerlory]

Nope, something like this: #19189 (comment)

Fixed by resetting index = 0 when out of bounds - this eliminates the bounds check while avoiding unnecessary backward scanning that index = len-1 would cause. Is this what you are looking for?