This missing op causes horrible template errors when attempting to use UInt4 swizzles.

Change-Id: I02f9381c1d4da45b1fc32e53ad9c7fd248e21673
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31273
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>