glsl/lower_precision: split out const lowering

glsl/lower_precision: split out const lowering

Some hw can narrow 32b const/uniform to 16b on load.. and in particular
lowering constants to 16b would break const->uniform lowering.  Allow
them to lower temps to 16b, while skipping consts.

Initially it is set to the same value as LowerPrecisionTemporaries, to
preserve the current behavior.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6189>