[dev.simd] simd, cmd/compile: add float -> int conversions

[dev.simd] simd, cmd/compile: add float -> int conversions

This CL also fixed some documentation errors in existing APIs.

Go defaults MXCSR to mask exceptions, the documentation is based on this
fact.

Change-Id: I745083b82b4bef93126a4b4e41f8698956963704
Reviewed-on: https://go-review.googlesource.com/c/go/+/724320
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>