SI-7006 Improve jump-elision code in GenASM

While working on SI-7006 I found a O(N*M) loop in jump-elision that
should be O(N). This commit clean that up. It also improves the
diagnostics in Members#removeBlock.
SI-7006 Recognize more jump only blocks

During ASM code emission we would recognize a block that consisted of
ICode-only artifacts (ENTER_SCOPE, EXIT_SCOPE, and LOAD_EXCEPTION)
followed by a jump. But we weren't using the same logic to recognize
all jump-only blocks. So jump-elision wasn't removing them. And that
in fact was why the ASM code emission had to do its special case.

This commit makes all jump-only block recognition use the same logic:
a jump-only block is one that has 0 or more ICode-only instructions
followed by a JUMP. It does't necessarily have to start with a JUMP.

There's now a debugWarning if the old NOP emitting code is triggered and
test t6102 is enhanced to error if that warning occurs.
SI-7006 Eliminate unreachable blocks

GenASM was doing a bunch of stuff to eliminate unreachable exception
handlers, but it was still leaving behind other unreachable blocks, for
instance a finally block associated with an exception handler that got
removed would still be left lying around. ASM would in turn turn those
into a big pile of NOPs, which just take up space uselessly.

This commit replaces all the logic for eliding exception handlers with
a single unreachable block remover that catches unused exception
handlers and a whole lot more.
SI-7006 Simplify jump-only block destination determination

With proper reachability analysis, the code for finding the final
destination of jump-only blocks was more complicated than needed. This
commit simplifies and speeds up that process using a standard
Tortoise and Hare algorithm on a Map from jump-only blocks to their
immediate destinations.

Test t7006 is increased a bit to make sure we don't get stuck on
infinite loops and to make sure we're deleting all but the essential
jump.
Merge pull request #2158 from JamesIry/master_7006

SI-7006 Eliminate NOPs and unreachable code