Subzero has unrelated types for constants and variables. Both are
operands to instructions, but we can't express this relationship at
the Nucleus level. We'd require a large number of Value to Operand
conversions. Instead, an Assign instruction is provided to convert
a Constant into a Value.

Bug swiftshader:12

Change-Id: Ie35a2cea3e485c4012ed949f92825a41caca3367
Reviewed-on: https://swiftshader-review.googlesource.com/7370Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>