The full 64x64->128 multiply comes up when using bits.Mul64.
The 64x64->64+overflow multiply comes up in unsafe.Slice when using
a constant length.

Change-Id: I298515162ca07d804b2d699d03bc957ca30a4ebc
Reviewed-on: https://go-review.googlesource.com/c/go/+/667175


Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>