The non-float variations of the mix builtin were conditioned to desktop
GLSL, but they are present in ESSL 3.1+.  This change also implements
constant folding of these builtins as exercised by dEQP.

Bug: angleproject:4300
Change-Id: Iec34de4cf370e00d67fd605148cd7848f9e122f8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2006809
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>