[LoopUnroll] Add multi-exit test which does not exit through latch.

This patch adds a new test for loop-unrolling with multiple exiting
blocks, where the latch does not exit, but the header does. This can
happen when the loop has not been rotated, e.g. due to minsize.

Inspired by the following end-to-end test, using -Oz
https://godbolt.org/z/fP6sna8qK

    bool foo(int *ptr, int limit) {
        #pragma clang loop unroll(full)
        for (unsigned int i = 0; i < 4; i++) {
            if (ptr[i] > limit)
            return false;
            ptr[i]++;
        }
        return true;
    }