gallivm: Return 0 for first active invocation when no invocations are active.

gallivm: Return 0 for first active invocation when no invocations are active.

gallivm doesn't actuially jump across branches where no invocations are
active, so my previous assertion about the exec mask being nonzero was
incorrect.  This means that we'll always use a defined invocation for the
various LLVMBuildExtractElements using the result value, which is an
improvement over my even the code before my cttz change that would use
undefined values for the element to be extracted.

Fixes: 8c2493d041c4 ("gallivm: Use cttz instead of a loop for first_active_invocation().")
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21142>