cmd/compile/internal/ssagen: add check for duplicate intrinsics
Add a check to ensure that intrinsics are not being overwritten.
Remove two S390X intrinsics that are being replaced by aliases and
are therefore ineffective.
Change-Id: I4187a169c14ca75c45a67f41a1d626d76b82bb72
Reviewed-on: https://go-review.googlesource.com/c/go/+/605479
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org>