This serves two purposes:
1. All rr::Variables held in these containers are destructed, preventing pointless materialization.
2. Frees memory that will never be used again.

Bug: b/135609394
Change-Id: Ia1e3e108d2594685320e9a1d96c1d47532f05c0c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33148Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>