We have no native way to swizzle out a nonzero component in a load, but
we can simply load extra components and do the swizzle in shader
instructions. This is inefficient, since it loads data to discard
immediately, but it's required for conformance in some edge cases.
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11123>
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