[dev.simd] cmd/compile, simd: add ClearAVXUpperBits

[dev.simd] cmd/compile, simd: add ClearAVXUpperBits

Intended for transitioning from AVX to SSE, this helps early
adopters benchmarking. The compiler should take care of that,
one day.

Change-Id: I9d7413f22f30f8dc0c632e8e806386d9ca8e8308
Reviewed-on: https://go-review.googlesource.com/c/go/+/701199
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: David Chase <drchase@google.com>