This moves some conversions to NIR that may get eliminated, and also
distinguishes gl_FragCoord.z/w loads at the shader info level so we don't
need to flag uses_src_depth/uses_src_w when only gl_FragCoord.xy get used
(as is typical). This reduces thread payload setup on many shaders.
Also, interestingly, blorp shaders stop reserving space for z/w despite
not putting them in the payload (since PS_EXTRA isn't filled out for z/w).
HSW shader-db is noise:
total instructions in shared programs: 9942649 -> 9942997 (<.01%)
instructions in affected programs: 143167 -> 143515 (0.24%)
total cycles in shared programs: 314768862 -> 314299112 (-0.15%)
cycles in affected programs: 62951452 -> 62481702 (-0.75%)
LOST: 44
GAINED: 26
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25190>
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