[dev.simd] cmd/compile, simd: add packed saturated u?int conversions

[dev.simd] cmd/compile, simd: add packed saturated u?int conversions

This CL should complete the conversions between int and uint.

Change-Id: I46742a62214f346e014a68b9c72a9b116a127f67
Reviewed-on: https://go-review.googlesource.com/c/go/+/698236
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: David Chase <drchase@google.com>
Reviewed-by: David Chase <drchase@google.com>