Refactor constant creation.

Refactor constant creation.

Distinguishing between Values and Constants complicates the creation
of constant vectors, and isn't necessary when using Subzero assign
operations to convert between them internally. Also, construct vector
constants from arrays of basic types.

Bug swiftshader:17

Change-Id: I9c03655ed18d5b4bd3797a252cd7f02793205254
Reviewed-on: https://swiftshader-review.googlesource.com/7713
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-on: https://swiftshader-review.googlesource.com/7650
Reviewed-by: Alexis Hétu <sugoi@google.com>