[CIR][CIRGen] Add more support for __cxa_rethrow

[bruteforceboy]

This is the second part of PR#1290, adding initial support for __cxa_rethrow.

So, the last PR did not support statements that come after the UnreachableOp from the rethrow, we just ignored them, e.g:

struct S {
  S() {}
};

void foo() {
  int r = 1;
  try {
    throw;
    S s;
  } catch (...) {
    ++r;
  }
}

This PR fixes this.

A few changes:

• rethrow statements split their block into multiple blocks.
• Tests with statements that come after the rethrow were added and old ones were modified.

Concern:

• The ScopeOp workaround still exists which I guess was one of the main concerns when we tried in the last PR.

As usual, I am open to discussions on this and how to approach it better -:)

[bcardosolopes]

Ready to go after nits get addressed.