i965/fs: Refactor try_emit_saturate

i965/fs: Refactor try_emit_saturate

v3: Since the fs backend can emit saturate as a separate instruction, there is
    no need to detect for min/max instructions and to rewrite the instruction tree
    accordingly. On the other hand, we don't need to emit a separate saturated
    mov either when the expression generating src can do saturate directly.
v4: Add can_do_saturate() check before enabling saturate modifer (Ken)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>