Metal: Fix missing mix(float,float,bool) overload ANGLE has an ANGLE_mix_bool overload when the last parameter is a bool but it is not needed when the other parameters are scalar. Bug: chromium:1493243 Change-Id: I6ea904ed63055b49cf3eaded475eafa607ca5976 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4995278 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>