This refactoring paves the way for the next patch to support
constructing RValues from Constant pointers as well.

Bug swiftshader:12

Change-Id: I26df3da13314de556dc1af40388278d144560b97
Reviewed-on: https://swiftshader-review.googlesource.com/7371Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>