intel/fs: Implement "demote to helper invocation"

intel/fs: Implement "demote to helper invocation"

The "demote" intrinsic works like "discard" but don't change the
control flow, allowing derivative operations to work.  This is the
semantics of D3D discard.

The "is_helper_invocation" intrinsic will return true for helper
invocations -- both the ones that started as helpers and the ones that
where demoted.  This is needed to avoid changing the behavior of
gl_HelperInvocation which is an input (so not expected to change
during shader execution).

v2: Emit the discard jump and comment why it is safe.  (Jason)
    Rework the is_helper_invocation() that was stomping f0.1.  (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>