panfrost/midgard: Use a union to manipulate embedded constants

panfrost/midgard: Use a union to manipulate embedded constants

Each instruction bundle can contain up to 16 constant bytes. The meaning
of those byte is instruction dependent: it depends on the instruction
native type (int, uint or float) and the instruction reg_mode (8, 16, 32
or 64 bit). Those different layouts can be exposed as a union to
facilitate constants manipulation.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3478>