If a store address has no definition, it must be a pointer argument.
Unlike stores to alloca's, these are not dead.

Change-Id: I66e50b14ffea1cfc61fc756b5545ab8ae28b4bf8
Reviewed-on: https://swiftshader-review.googlesource.com/8573Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>