cmd/compile: remove duplicate const logic from typecheck
Now that we always use types2 to validate user source code, we can
remove the constSet logic from typecheck for detecting duplicate
expression switch cases and duplicate map literal keys. This logic is
redundant with types2, and currently causes unified IR to report
inappropriate duplicate constant errors that only appear after type
substitution.
Updates #42758.
Change-Id: I51ee2c5106eec9abf40eba2480dc52603c68ba21
Reviewed-on: https://go-review.googlesource.com/c/go/+/390474
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>