Mostly trivial; exception is RoundEven, for which I borrowed the
implementation from the GLES side.

Test: dEQP-VK.glsl.operator.common_functions.*
Bug: b/127804400
Change-Id: I889c3750ebdbc078723890488cf44d89ea77103b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26628Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>