radv: fix unused non-xfb shader outputs not being removed

radv: fix unused non-xfb shader outputs not being removed

It was not taken into account that without Offset decoration
the output is not written into XFB.

Aside from eliminating more outputs this change prevents gl_PerVertex
builtins generated by glslang from being kept alive in case when XFB
is enabled. Keeping such outputs alive may upset a driver.

VUID-StandaloneSpirv-Offset-04716:
    "Only variables or block members in the output interface decorated
    with Offset can be captured for transform feedback, and those
    variables or block members must also be decorated with XfbBuffer
    and XfbStride, or inherit XfbBuffer and XfbStride decorations from
    a block containing them"

Additional info about glslang behavior could be found at:
 https://github.com/KhronosGroup/glslang/issues/1526

Fixes: e95531e101f0ba61d28195fe38414e411bf418b3
("radv: fix gathering XFB info if there is dead outputs")

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24318>