Treat OpUndef identically to OpConstantNull for now.

Change-Id: I721f2bf5e053d6a8d6f563272b1d44b6636a157c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27529Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>