JIT: Detect degenerate switches in switch recognition

[jakobbotsch]

Fix #121691

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Pull request overview

This PR addresses issue #121691 by detecting degenerate switches in the JIT's switch recognition phase. A degenerate switch occurs when all test paths lead to the same target block, meaning there's only one unique successor instead of two.

Key Changes:

• Dynamically calculates the number of unique successors based on whether blockIfTrue equals blockIfFalse
• Updates the BBswtDesc constructor call to use the computed numSuccs value instead of a hardcoded 2

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @EgorBo

[EgorBo]

presumably if all successors target the same block we can remove the switch entirely, but I guess here it's just some stress artifact

[jakobbotsch]

presumably if all successors target the same block we can remove the switch entirely, but I guess here it's just some stress artifact

No, this actually happens in real code. But we have a later flowgraph optimization that removes the switch, so I didn't bother doing anything more complicated here.

[jakobbotsch]

/ba-g iOS deadletter