nir/opt_shrink_vectors: Round to supported vec size

nir/opt_shrink_vectors: Round to supported vec size

The set of supported vector sizes in NIR has holes in it. For example, we
support vec5 and vec8, but not vec6 or vec7. However, this pass did not take
that into account, and would happily shrink a vec8 down to a vec7, causing NIR
validation to fail. Instead, the pass should round up to the next supported
vector size.

Fixes NIR validation fail in OpenCL's test_basic hiloeo subtest.

v2: Clamp -> round rename.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17194>