The spec states that opaque types can have a None size instead of a constant integer size.

Bug: b/148401179
Change-Id: I52eeabe2336775643787599890857169825f3230
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45353Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>