Avoids collisions in the dynamic sampling function cache.

Change-Id: I41b5e5723c3f23f67722e006bdbc603ba1ba18f5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31029Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>