i965/fs: Don't generate saturates over existing variable values.

i965/fs: Don't generate saturates over existing variable values.

Fixes a crash in http://workshop.chromeexperiments.com/stars/ on i965,
and the new piglit test glsl-fs-clamp-5.
We were trying to emit a saturating move into a uniform, which the code
generator appropriately choked on.  This was broken in the change in
32ae8d3b321185a85b73ff703d8fc26bd5f48fa7.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57166
NOTE: This is a candidate for the 9.0 branch.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>