i965: Free dead GLSL IR one last time.

i965: Free dead GLSL IR one last time.

While working on NIR's memory allocation model, I realized the GLSL IR
memory model was broken.

During glCompileShader, we allocate everything out of the
_mesa_glsl_parse_state context, and reparent it to gl_shader at the end.

During glLinkProgram, we allocate everything out of a temporary context,
then reparent it to the exec_list containing the linked IR.

But during brw_link_shader - the driver's final opportunity to do
lowering and optimization - we just allocated everything out of the
permanent context given to us by the linker.  That memory stayed
forever.

Notably, passes like brw_fs_channel_expressions cause us to churn the
majority of the code, so we really want to free dead IR here.

Saves 125MB of memory when replaying a Dota 2 trace on Broadwell.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>