[LoopUnroll] Clean up remarks for unroll remainder

The optimisation remarks for loop unrolling with an unrolled remainder looks something like:

test.c:7:18: remark: completely unrolled loop with 3 iterations [-Rpass=loop-unroll]
            C[i] += A[i*N+j];
                 ^
test.c:6:9: remark: unrolled loop by a factor of 4 with run-time trip count [-Rpass=loop-unroll]
        for(int j = 0; j < N; j++)
        ^
This removes the first of the two messages.

Differential revision: https://reviews.llvm.org/D38725

llvm-svn: 316986