ac/nir: Return frag_coord as integer.

To preserve the invariant that nir ssa defs are integers or pointers
in LLVM.

CC: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit 82075e3c)