Even if only variables access flags are changed, the existing NIR
infrastructure expects metadata to be explicitly preserved, so do
that.  Don't care about avoiding preserve to be called twice since the
cost is negligible.

This scenario can be triggered by dead variables, and also by other
intrinsics that read the variables -- but not cause progress to be
made when processing the intrinsics.

Fixes: f2d0e48d

 "glsl/nir: Add optimization pass for access flags"
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>