[LoopUnroll][NFC] Remove redundant canPeel check

We check `canPeel` twice: when evaluating the number of iterations to be peeled
and within the method `peelLoop` that performs peeling. This method is only
executed if the calculated peel count is positive. Thus, the check in `peelLoop` can
never fail. This patch replaces this check with an assert.

Differential Revision: https://reviews.llvm.org/D44919
Reviewed By: fhahn

llvm-svn: 328615