This saves a bit of bandwidth when we're not going to use the value.
Improves renderpass times across 4 affected traces I tested (bioshock,
stranded deep, transport fever, and godot material testers) on sysmem by
.3% +/- .1%.
A similar change for avoiding stencil reads showed no change on the one
app affected among all of our renderdoc traces, so that's left out.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34964>
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