Eliminate assign operations for constants.

Eliminate assign operations for constants.

We were using Ice::Variable as the implementation type for Reactor's
abstract Value class only because shuffle operations required them.

Bug swiftshader:24

Change-Id: If87ab5f0b0bca5fbffe2df250ed03b7a1b1490c6
Reviewed-on: https://swiftshader-review.googlesource.com/8258
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>