cmd/compile: do not add invalid key to constSet

After CL 272654, the compiler now use go/constant.Value to represent
constant nodes. That makes ir.ConstantValue requires node type to
correctly return value for untyped int node. But untyped int node can
have nil type after typechecked, e.g: using int value as key for
map[string]int, that makes the compiler crashes.

To fix it, just don't add the invalid key to constSet, since when
it's not important to report duplicated keys when they aren't valid.

For #43311
Fixes #44432

Change-Id: I44d8f2b95f5cb339e77e8a705a94bcb16e62beb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/294034


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>