The bound parameter allows us to prevent allocations from crossing
particular boundaries (typically 128-bit boundaries). For 16-bit, we
don't want to cross 64-bit boundaries, in order to keep swizzles
possible to encode. We already handle this for 16-bit destinations, but
it _also_ needs to be (redundantly) handled for 16-bit sources, in case
types don't match (for example, with a vectorized size conversion
instruction).

Fixes a few newer dEQP fails.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <mesa/mesa!8282>