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

This is a reland of commit 262311772b2128ef778d0a1d189f0c6c51780132

When running with sound null safety, reducing a logical expression to
the right hand side if the left hand side is constant and does not short
circuit is valid, as the right hand side is guaranteed not to evaluate
to null.

In other modes, however, the right hand side may evaluate to null.
Thus, in thos modes, we return the original node if the RHS is not a
constant boolean value, so that the operator can perform whatever null
checking is required.

Fixes: https://github.com/dart-lang/sdk/issues/54029

Original change's description:
> 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>

TEST=pkg/vm/test/transformations/unreachable_code_elimination
     pkg/vm/test/transformations/vm_constant_evaluator

Change-Id: Ia51b7c5f3b51f57a6a306551fe74b47e0cba3c23
Cq-Include-Trybots: luci.dart.try:vm-aot-linux-release-x64-try,vm-aot-mac-release-arm64-try,vm-kernel-precomp-linux-release-x64-try,vm-kernel-linux-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/335828
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>

https://dart.googlesource.com/sdk/+/4bedb142d3de63cc214cc7e53bf4ecf6debc96fd