Add an assert concerning branch to BBJ_CALLFINALLY blocks (#55858)

Add an assert concerning branch to BBJ_CALLFINALLY blocks (#55858)

In the FEATURE_EH_CALLFINALLY_THUNKS case, BBJ_CALLFINALLY blocks live
in the EH region enclosing the `try` block that needs to call the finally.
However, we need all flow to the BBJ_CALLFINALLY to come from that try block;
we don't want flow optimizations to otherwise branch directly to this
BBJ_CALLFINALLY block. Add an assert to verify this is the case. The assert
also covers the non-FEATURE_EH_CALLFINALLY_THUNKS case.