intel/compiler: fix dynamic alpha-to-coverage handling

intel/compiler: fix dynamic alpha-to-coverage handling

Got the wrong logic operation. Let's reuse the nicer NIR builder
helper.

Fixes a bunch of KHR-GL46.sample_variables.mask.rgba8.*.samples*.mask*

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: fd7debc8bb ("intel/fs: make alpha_to_coverage a tristate")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9568
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24716>