This test was silently failing since a long time because it failed to include stdlib.h (as it's running in a freestanding environment). However, because we used just not clang_cc1 instead of the verify mode, this regression was never noticed and the test was just always passing.
This adds -ffreestanding to the invocation, so that tmmintrin.h doesn't indirectly include mm_malloc.h, which in turns includes the unavailable stdlib.h. We also run now in the -verify mode to prevent that we silently regress again.
I've also updated the test to no longer check the return value of _mm_alignr_epi8 as this is also causing it to fail (and it's not really the job of this test to test this).