cmd/compile: don't convert non-Go-constants to OLITERALs

cmd/compile: don't convert non-Go-constants to OLITERALs

Don't convert values that aren't Go constants, like
uintptr(unsafe.Pointer(nil)), to a literal constant. This avoids
assuming they are constants for things like indexing, array sizes,
case duplication, etc.

Also, nil is an allowed duplicate in switches. CTNILs aren't Go constants.

Fixes #28078
Fixes #28079

Change-Id: I9ab8af47098651ea09ef10481787eae2ae2fb445
Reviewed-on: https://go-review.googlesource.com/c/151320
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>