cmd/compile: handle constant-folding of an out-of-range jump table index

cmd/compile: handle constant-folding of an out-of-range jump table index

The input index to a jump table can be out of range for unreachable code.

Dynamically the compiler ensures that an out-of-range index can never
reach a jump table, but that guarantee doesn't extend to the static
realm.

Fixes #64826

Change-Id: I5829f3933ae5124ffad8337dfd7dd75e67a8ec33
Reviewed-on: https://go-review.googlesource.com/c/go/+/552055
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>