Blocks insert a leading `nop` instruction in order to execute a "block
call" tracepoint. Block compilation unconditionally inserts a leading
`nop` plus a label after the instruction:

  https://github.com/ruby/ruby/blob/641f15b1c6bd8921407a1f045573d3b0605f00d3/prism_compile.c#L6867-L6869

This `nop` instruction is used entirely for firing the block entry
tracepoint.  The label exists so that the block can contain a loop but
the block entry tracepoint is executed only once.

For example, the following code is an infinite loop, but should only
execute the b_call tracepoint once:

```ruby
-> { redo }.call
```

Previous to this commit, we would eliminate the `nop` instruction, but
only if there were no other jump instructions inside the block.  This
means that the following code would still contain a leading `nop` even
though the label following the `nop` is unused:

```ruby
-> { nil if bar }
```

```
== disasm: #<ISeq:block in <main>@test.rb:1 (1,2)-(1,17)> (catch: FALSE)
0000 nop                                                              (   1)[Bc]
0001 putself                                [Li]
0002 opt_send_without_block                 <calldata!mid:bar, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0004 branchunless                           8
0006 putnil
0007 leave                                  [Br]
0008 putnil
0009 leave                                  [Br]
```

This commit checks to see if the label inserted after the `nop` is
actually a jump target.  If it's not a jump target, then we should be
safe to eliminate the leading `nop`:

```
> build-master/miniruby --dump=insns test.rb
== disasm: #<ISeq:<main>@test.rb:1 (1,0)-(1,17)>
0000 putspecialobject                       1                         (   1)[Li]
0002 send                                   <calldata!mid:lambda, argc:0, FCALL>, block in <main>
0005 leave

== disasm: #<ISeq:block in <main>@test.rb:1 (1,2)-(1,17)>
0000 putself                                                          (   1)[LiBc]
0001 opt_send_without_block                 <calldata!mid:bar, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 branchunless                           7
0005 putnil
0006 leave                                  [Br]
0007 putnil
0008 leave                                  [Br]
```

We have a test for b_call tracepoints that use `redo` here:

  https://github.com/ruby/ruby/blob/aebf96f371c8d874398e0041b798892e545fa206/test/ruby/test_settracefunc.rb#L1728-L1736