The only elect function we'll need to implement for Pastel 1.0.

Used by memory barrier tests, but not tested on its own.

Bug: b/131667233
Change-Id: I5309fa6b49c49091226fbaaca4e46363f6dff534
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30208
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>