JIT: Remove fallthrough checks in Compiler::TryLowerSwitchToBitTest

[amanasifkhalid]

Part of #107749, and prerequisite to #107634. If we intend to run block layout after lowering, then we shouldn't bother checking for fallthrough during switch lowering -- instead, rely on block layout to create fallthrough from the bit test if it is beneficial.

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[amanasifkhalid]

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. Modest diffs -- this change is creating instances where the BBJ_COND block containing the bit test is not followed by either of its successors. Once we move block layout to the backend, we should be able to fix this when it makes sense to have fallthrough.

[amanasifkhalid]

ping @AndyAyersMS