JIT: block throw helper merges for first block of a try (#34039)

JIT: block throw helper merges for first block of a try (#34039)

Otherwise we may create a branch into the middle of a try. We could fix the
transform, but if the first block of a try has a throw helper call, the rest
of the try will subsequently be removed, so merging is not all that
interesting.

Addresses an issue that came up in #33924.