JIT: fix filter liveness computation (#23044)

JIT: fix filter liveness computation (#23044)

When a filter is finished executing, control can logically pass to the
associated handler, any enclosing handler or filter, or any finally or fault
handler nested within the associated try. This is a consequence of two-pass EH.

The jit was not propagating liveness from the nested handlers, which lead to a
live object being collected inadvertently.

This change updates `fgGetHandlerLiveVars` to find the nested handlers and
merge their live-in into the filter block live sets.

Because these implicit EH flow edges can create cycles in the liveness dataflow
equations, the jit will also now always iterate liveness when it sees there is
exception flow, to ensure livness reaches the appropriate fixed point.

Added test case.

Closes #22820.