i965/fs: Fix off-by-one in saturate propagation.

ip needs to be initialized to start_ip - 1, since the first thing in the
main loop is ip++. Otherwise we would incorrectly propagate the saturate
from the mov to the mad:

   mad     a, b, c, d
   mov.sat x, a
   add     y, z, a

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/i965/brw_fs_saturate_propagation.cpp

@@ -32,7 +32,7 @@ static bool
 opt_saturate_propagation_local(fs_visitor *v, bblock_t *block)
 {
    bool progress = false;
-   int ip = block->start_ip;
+   int ip = block->start_ip - 1;
 
    for (fs_inst *inst = (fs_inst *)block->start;
         inst != block->end->next;