Fix bug and leak in relooper merge consecutive blocks

[hobby8]

Fixes in Relooper merge consecutive blocks:

1. Entry block getting removed when it is part of a loop:

bb1->AddBranchTo(bb2, nullptr);
bb1->AddBranchTo(bb3, ...);
bb2->AddBranchTo(bb1, nullptr);
bb3->AddBranchTo(bb4, nullptr);
relooper.AddBlock(bb1);
relooper.AddBlock(bb2);
relooper.AddBlock(bb3);
relooper.AddBlock(bb4);
relooper.Calculate(bb1);

2. Branches memory leak

[kripken]

Thanks @hobby8! This looks right to me.

1. Have you joined the wasm w3c group?
2. Please add a test for this. There are a bunch of relooper tests in test/example/ and this can be added to one of them, I think.

[hobby8]

Opted for a minimal new test that exercises mostly the CFG API, hope that's OK.

[hobby8]

I forgot to mention joining the CG. If you prefer to not maintain an additional test, I can look at the existing ones in more detail.

[kripken]

Great, thanks @hobby8! And test looks very good.