cmd/compile: optimize SIMD IsNaN.Or(IsNaN)

IsNaN's underlying instruction, VCMPPS (or VCMPPD), takes two
inputs, and computes either of them is NaN. Optimize the Or
pattern to generate two-operand form.

This implements the optimization mentioned in CL 733660.

Change-Id: I13943b377ee384864c913eed320763f333a03e41
Reviewed-on: https://go-review.googlesource.com/c/go/+/733680
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>