i965/vec4: Optimize sqrt+inv into rsq.

i965/vec4: Optimize sqrt+inv into rsq.

Transform

   sqrt a, b
   rcp  c, a

into

   sqrt a, b
   rsq  c, b

In most cases the sqrt's result is still used, so the improvement here
is that we've broken a dependency between these instructions. Leads to
80 fewer INV instructions and 80 more RSQ.

Occasionally the sqrt's result is no longer used, leading to:

instructions in affected programs:     5005 -> 4949 (-1.12%)

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>