simd/archsimd: make IsNaN unary

simd/archsimd: make IsNaN unary

Currently, the IsNan API is defined as x.IsNan(y), which returns
a mask to represent, for each element, either x or y is NaN.
Albeit closer to the machine instruction, this is weird API, as
IsNaN is a unary operation. This CL changes it to unary, x.IsNaN().
It compiles to VCMPPS $3, x, x (or VCMPPD). For the two-operand
version, we can optimize x.IsNaN().Or(y.IsNaN()) to VCMPPS $3, x,
y (not done in this CL).

While here, change the name to IsNaN (uppercase both Ns), which
matches math.IsNaN.

Tests in the next CL.

Change-Id: Ib6e7afc2635e6c3c606db5ea16420ee673a6c6d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/733660
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>