Merge basic blocks where possible when generating LLVM IR.

In `codegen_assert_terminator` we decide if a BB's successor is a
candidate for merging, which requires that it be the only successor, and
that it only have one predecessor. That result then gets passed down,
and if it reaches `funclet_br` with the appropriate BB characteristics,
then no `br` instruction is issued, a `MergingSucc::True` result is
passed back, and the merging proceeds in `codegen_block`.

The commit also adds `CachedLlbb`, a new type to help keep track of
each BB that has been merged into its predecessor.

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