This prepares for simplifying assigning the same scalar to all elements.

Bug: b/146224130
Change-Id: I8499c671f98090ae38fd12bab59f30ef360eff69
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39508
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>