Identify when consecutive branches of if/else have the same contents and merge them

Corollary of #10283.

It isn't uncommon for a method body to look something like this:

if (!ready) {
  return;
}

if (input.isEmpty()) {
  return;
}

for (Item i : input) {...}

#10240 will get this code to something like

if (!ready) {
  return;
} else if (input.isEmpty()) {
  return;
} else {
  for (Item i : input) {...}
}

When we see consecutive branches with the same body, their conditions can be merged with ||, and short circuiting will avoid evaluating more than we should:

if ((!ready) || (input.isEmpty())) {
  return;
} else {
  for (Item i : input) {...}
}

We don't generally have a "are these two JNodes equal" as far as I'm aware - #10283 will also need something like this (or also a "prefix"/"suffix" check for blocks).

With #10248, we then would have the "then" be an empty block, so the conditiona can be flipped, and the method body is (with the ! distributed)

if (ready && !input.isEmpty() {
  for (Item i : input) {...}
}

Start by reading the transformations discussed in #10283, #10240, and #10248, then trace how the compiler represents and compares JNodes and branch blocks. Done means consecutive if/else branches with equivalent bodies can be merged using short-circuiting while preserving evaluation order and behavior.