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

This is a reland of commit 92bf76d9e86c65564952cdd7e9e53ad76b501dce

In the original CL, the changes to the UCE assumed all
SwitchStatements were exhaustive. Now if a SwitchStatement isn't
explicitly or implicitly (via a default case) exhaustive and no case
matches the tested constant, the SwitchStatement is properly removed.

In addition, if a guaranteed to match case is found, more than one
cases remain (thus the SwitchStatement is not removed or replaced with
the single case's body), and the default case is removed, then the
resulting SwitchStatement is marked as explicitly exhaustive, as this
serves as a signal to backends that they do not need to handle the
possibility of no case matching in the absence of a default case.

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>

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
Cq-Include-Trybots: luci.dart.try:vm-aot-linux-release-x64-try,vm-aot-mac-release-arm64-try
Change-Id: I557ca933808012e670e306f2d880221a0d7dd670
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/334224
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>