Implement non-optimized version. Should generalize and optimize code in
UShort4::UShort4(RValue<Float4> cast, bool saturate) so we can use it
here.

Bug: b/145754674
Change-Id: I16a24dab2979bb97d2d5f36a406aeb62aeb976e8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41928Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>