glsl: Add a method to get precision from a deref instruction

glsl: Add a method to get precision from a deref instruction

Adds ir_dereference::precision(). For a normal variable dereference,
the precision comes from the variable. For a record member it comes
from the field within the record. For an array it can come from
either, depending on where the underlying array is stored. The method
recursively walks the derefs until it finds one of the first two.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3885>