i965/gen7: Handle atomic instructions from the FS back-end.

i965/gen7: Handle atomic instructions from the FS back-end.

This can deal with all the 15 32-bit untyped atomic operations the
hardware supports, but only INC and PREDEC are going to be exposed
through the API for now.

v2: Represent atomics as GLSL intrinsics.  Add support for variably
    indexed atomic counter arrays.  Fix interaction with fragment
    discard.
v3: Add comment on why we don't need to assign uniform storage for
    atomic counters.

Reviewed-by: Paul Berry <stereotype441@gmail.com>