When every dst_sel of a sampled view selects constant 0 or 1, a sample
or gather returns that constant whatever the texel, filter, LOD or
coordinates. Such placeholder views can have a dimension the static
runtime image interface has no mode for (a 3D view under a 2D sample,
a 1D view under a 2D gather), and a 3D one paired with an unnormalized
sampler fails the unnormalized proof.
A direct sampled image whose accesses are all non-compare samples or
gathers gets three constant runtime modes (float, uint, sint). The mode
is selected when all four dst_sel are 0 or 1, unless the view is FMASK,
depth bits, converted or sRGB-decoded. In it the emitter returns each
channel's constant (the dmask channel's, in all four texels, for a
gather) from the image's swizzle specialization constants, and the
image is left unbound. ProveUnnormalized skips the mode after the
run-time-selected check, so table roots still throw.
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