Just a refactoring to improve syntax and avoid non-const references.

Also don't assume SIMD types are 4-wide.

Also add [[fallthrough]] cases found in the process.

Bug: b/143351714
Bug: b/142661203
Change-Id: I12a0178338ce4c711bcbf62825d230580f3e92f0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40288
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>