cmd/compile: don't optimize away a panicing interface comparison

cmd/compile: don't optimize away a panicing interface comparison

We can't do direct pointer comparisons if the type is not a
comparable type.

Fixes #76008

Change-Id: I1687acff21832d2c2e8f3b875e7b5ec125702ef3
Reviewed-on: https://go-review.googlesource.com/c/go/+/713840
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>