Revert "[pkg/vm] Handle switch statements in unreachable code eliminator."

This reverts commit 92bf76d9e86c65564952cdd7e9e53ad76b501dce.

Reason for revert: broken tests on AOT trybots

Original change's description:
> [pkg/vm] Handle switch statements in unreachable code eliminator.
>
> Namely, if the tested expression for a switch statement is constant,
> then we can remove any constant cases where the constants differ,
> and if all but a single case is removed, we can replace the switch
> with the case body.
>
> If constant functions are not enabled, then getters annotated with
> @pragma("vm:platform-const") are still evaluated with the constant
> function evaluation machinery, but only those and no others (including
> any functions called within an annotated getter). This way, functions
> can be annotated with @pragma("vm:platform-const") without having to
> rewrite them to be a single returned expression.
>
> TEST=pkg/vm/test/transformations/unreachable_code_elimination
>      pkg/vm/test/transformations/vm_constant_evaluator
>
> Issue: https://github.com/dart-lang/sdk/issues/50473
> Issue: https://github.com/dart-lang/sdk/issues/31969
> Change-Id: Ie290d2f1f469326238d66c3d9631f8e696685ff0
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332760
> Commit-Queue: Tess Strickland <sstrickl@google.com>
> Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
> Reviewed-by: Alexander Markov <alexmarkov@google.com>

Issue: https://github.com/dart-lang/sdk/issues/50473
Issue: https://github.com/dart-lang/sdk/issues/31969
Change-Id: I4340874793da19ce892a19bcf0542c24e0ee65d4
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/334600
Commit-Queue: Siva Annamalai <asiva@google.com>
Auto-Submit: Tess Strickland <sstrickl@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>