glsl/cse: Maintain a list of free ae_entry objects

glsl/cse: Maintain a list of free ae_entry objects

The CSE algorithm will continuously allocate new ae_entry objects.  As
each new basic block is exited, all of the previously allocated objects
are dumped.  Instead, put them in a free list and re-use them in the
next basic block.  Reduce, reuse, recycle!

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>