go/constant: fix complex != unknown comparison
By the contract of Compare, if one operand is Unknown, the result must be
false.
Fixes #75137
Change-Id: I56420fae808395f89769f5e5d448f9e1df9a622f
GitHub-Last-Rev:
858ba89a91bf966223975541af739ab0ab977075
GitHub-Pull-Request: golang/go#75140
Reviewed-on: https://go-review.googlesource.com/c/go/+/698955
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>