st/mesa: improved is_interleaved_arrays() checking

st/mesa: improved is_interleaved_arrays() checking

Check that the difference in array pointers/offsets from the 0th
array are less than the stride, for both VBOs and user-space arrays.
Previously, we were only doing this for the later.

This tightens up the interleaved array test and fixes a problem with
the llvmpipe driver where we were creating way too many vertex fetch
variants only because the pipe_vertex_element::src_offset values were
changing frequently.  This change results in a 5x speed-up for one of
the viewperf tests.

Also, clean up the function to make it easier to understand.