Support assigning constants to values.

Support assigning constants to values.

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/7370
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>