mesa: make struct in gl_program a union and remove FIXME

mesa: make struct in gl_program a union and remove FIXME

Now that the classic drivers that were mixing the use of these asm
and glsl shader fields are gone we can finally use a union here.

This basically reverts commit 9d99dc4bc1fd but also moves a
read of IsPositionInvariant inside an arb asm only code block
for safety.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14059>