Update 0251 (simd) to reflect what was discussed in core team. (#1017)
Update 0251 (simd) to reflect what was discussed in core team. (#1017)

* Update 0251 (simd) to reflect what was discussed in core team.

Specifically, make permuting indexes wrap on vector length (even for SIMD3), and make min/max free functions instead of statics.

apple/swift-evolution,Nirma/swift-evolution,hartbit/swift-evolution,abertelrud/swift-evolution,natecook1000/swift-evolution,hartbit/swift-evolution,ddunbar/swift-evolution,DevAndArtist/swift-evolution,anandabits/swift-evolution,Nirma/swift-evolution,apple/swift-evolution,anandabits/swift-evolution,natecook1000/swift-evolution,abertelrud/swift-evolution,DevAndArtist/swift-evolution,ddunbar/swift-evolution