Fix a bug causes incorrect pla generation when input is `?` (backport #2113) (#2118)

* Fix a bug causes incorrect pla generation when input is `?` (backport #2113) (#2118)Add a test case to demonstrate the bug found in #2112

(cherry picked from commit 89679a3c)

* Test case rework

(cherry picked from commit 0ca4ce23)

* Fix #2112, handle `?->1` case

(cherry picked from commit 189da982

)

* Remove trailing comma

Co-authored-by: mergify-bot <noreply@mergify.io>
Co-authored-by: Jack Koenig <koenig@sifive.com>