spirv: Use correct type for sampled images

spirv: Use correct type for sampled images

We should use the result type of the OpSampledImage opcode, rather than
the type of the underlying image/samplers.

This resolves an issue when using separate images and shadow samplers
with glslang. Example:

    layout (...) uniform samplerShadow s0;
    layout (...) uniform texture2D res0;
    ...
    float result = textureLod(sampler2DShadow(res0, s0), uv, 0);

For this, for the combined OpSampledImage, the type of the base image
was being used (which does not have the Depth flag set, whereas the
result type does), therefore it was not being recognised as a shadow
sampler. This led to the wrong LLVM intrinsics being emitted by RADV.

Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Cc: "17.2 17.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>