[dev.simd] cmd/compile, simd: reorder PairDotProdAccumulate

[dev.simd] cmd/compile, simd: reorder PairDotProdAccumulate

This CL reorderes the param order of PairDotProdAccumulate family to be
dotprod(x, y) + z instead of the old dotprod(y, z) + x.

This CL also updates some documentation of other ML Ops.

This CL added a test to test the behavior is correct.

This CL is partially generated by CL 688115.

Change-Id: I76a6ee55a2ad8e3aff388d7e4fa5218ec0e4800d
Reviewed-on: https://go-review.googlesource.com/c/go/+/688095
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>