Disallow assigning to an RValue.

Change-Id: I539fcaa5b7853ac618f3d27027d3357bc6e620ba
Reviewed-on: https://swiftshader-review.googlesource.com/1960
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>