[polly] Fix i8 alignment in datalayout of lit test

Two lit test used overaligned i8, without the test case actually
depending on i8 alignment.
Change the datalayout string to use naturally aligned i8,
preparing for the upcoming requirement of naturally aligned i8.

GitOrigin-RevId: f287c1d3153e996502368f4be4f367dca65b03c7
2 files changed