“Holp” is the archaic past tense of “help”, which was formerly a strong verb (a verb that inflects by changing its vowel) like freeze/froze or ride/rode. The old form was obsolete in ordinary language by Shakespeare’s day, but it was kept alive through its use in biblical and liturgical texts, and it appears several times in the works of Shakespeare. Here are a couple of other instances (as printed in the First Folio):
Sicinius. Sir, how com’st that you haue holpe
To make this rescue?
Coriolanus, act III, scene I.
Worcester. And that same greatness too, which our owne hands
Have holpe to make so portly.
Henry IV Part I, act I, scene III.
In Titus Andronicus, the ending “’st” is second-person singular: without the elision, the word would be “holpest”. Saturninus is addressing himself to Titus in this soliloquy, accusing Titus of having helped him become emperor only to become the power behind the throne, and so the pronoun he uses for Titus is “thou”.
There is another appearance of the word in Richard III:
Duchess of York. I had a Richard too, and thou didst kill him;
I had a Rutland too, thou holp’st to kill him.
Richard III, act IV, scene 4.
In this speech, “holp’st” is parallel to “didst”, hence both are second-person past tense verb forms. (In the early Quarto and First Folio editions, the word was printed as “hop’st”, but that must have been a mistake by the scribe or compositor, since Edmund, Earl of Rutland, had been killed at the battle of Wakefield, before the beginning of the play. The later Quarto editions of the play have “holp’st”.)