nir/gather_info: Look for uses of helper invocations

nir/gather_info: Look for uses of helper invocations

The one obvious omission here is gl_HelperInvocation itself.  However,
the spec doesn't require that we generate then when gl_HelperInvocation
is used, it merely mandates that we report them if they are there.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>