I just want to expound on the answer by JamesK.
There are at least two reasons you shouldn't expect song lyrics to follow grammar rules:
Songs follow certain rhythm and rhyming schemes. Lyricists will often bend the rules of English to make a song fit their rhythm, or force a rhyme.
Songwriters often use informal English to set the tone for the speaker. Music is supposed to convey emotion. People in love don't speak to one another as if they are giving a formal talk at a conference. Folk singers, country singers, and rap singers try to portray the kind of people they are singing about, complete with the inelegant street language they so often use.
A few weeks ago, someone pointed me to a Bob Dylan lyric that is sprinkled with good examples. Remember, Bob Dylan was recently awarded the Nobel Prize for literature! But many of his lyrics would make a 3rd-grade teacher get out a red pen and start marking up all the "improper" English:
ain’t no ^wouldn't be any use to sit and wonder why, babe
don’t^doesn't matter, anyhow
An’ ^And it ain’t no^wouldn't be any use to sit and wonder why, babe
If you don’t know by now
When your rooster crows at the break of dawn
Look out your window and I’ll be gone
You’re the reason I’m
trav’lin’ ^traveling on
Don’t think twice, it’s all right
It ain’t no ^There isn't any use in turnin’ ^turning on your light, babe
That light I never
An’ ^And it ain’t no ^there isn't any use in turnin’ ^turning on your light, babe
I’m on the dark side of the road
I ain’t sayin’ ^I'm not saying you treated me unkind
You could have done better but I don’t mind
just kinda ^only wasted my precious time
But don’t think twice, it’s all right
(To see the full original in its unedited form, click here.)
There, I've made Bob Dylan's English conform to the grammar books – but I've practically ruined his song in the process! I've removed rhymes, I've disrupted the rhythm, I've cut out parallelism, and (perhaps most importantly), I've stripped mournful tone out of the main character's soul.
In the original, I can sense that the spurned man probably has unkempt hair and is wearing faded blue jeans while driving away in his 10-year-old vehicle. In my version, he might well have a funeral director's perfectly parted hair, along with a charcoal gray suit complemented by a conservative red necktie as he rides away in a brand-new, white Mercedes.
So, the next time you see a song lyrics that uses something non-standard, like "I was gonna love you 'til I's dead," just remember the immortal words of Bob Dylan:
Don't think twice, it's all right.