Stop space-leaking iris_compiled_shader
Each time we make a new shader variant (iris_compiled_shader), we add it to the hash table. These are never deleted, until the entire context is eliminated.
This means we'll continue wasting more and more memory. i965 "solves" this in the worst way possible by pitching all shaders when it reaches 2000 of them, which penalizes some games like GRID Autosport harshly.