glsl/linker: Free any IR discarded by optimization passes.

glsl/linker: Free any IR discarded by optimization passes.

Previously, IR for a linked shader was allocated directly out of the
gl_shader object - meaning all of it lived as long as the shader.

Now, IR is allocated out of a temporary context, and any -live- IR is
reparented/stolen to (effectively) the gl_shader.  Any remaining IR can
be freed.

NOTE: This is a candidate for the 7.9 branch.