CodeGen: Remove dead code in TailDuplicate

I noticed that this isn't covered by our existing tests and spent some
time trying to come up with an example it actually hits. I tried hand
rolling something based on the explanation in the comment, but couldn't
get anything that didn't abort tail duplication earlier for one reason
or another.

Then, I tried cranking tail-dup-size cranked up so this would fire
more and ran a bootstrap of clang and the nightly test suite - those
don't hit this either.

This reverts r132816 and replaces it with an assert.

llvm-svn: 265347