nir: Add read_invocation_cond_ir3 intrinsic

nir: Add read_invocation_cond_ir3 intrinsic

On qualcomm, we have shared registers similar to SGPR's on AMD. However,
there is no readlane or readfirstlane primitive. shared registers can
only be written to when just one lane is active. This means that we have
to lower readInvocation(val, id) to something like:

if (gl_SubgroupInvocation == id) {
    scalar_reg = val;
}

return scalar_reg;

However it's a bit difficult to actually get the value of
gl_SubgroupInvocation in the backend, because for compute it requires
some calculations and we don't have any CSE support in the backend. This
intrinsic lets us turn it into
"readInvocationCond(val, id == gl_SubgroupInvocation)" in NIR at which
point the backend code generation is a lot easier.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6752>