on the break instruction operands. This involves a new TZIR instruction,
br_block_flat, which represents a break instruction where the operand is
the result of a flat block. See the doc comments on the instructions for
more details.

How it works: when adding break instructions in semantic analysis, the
underlying allocation is slightly padded so that it is the size of a
br_block_flat instruction, which allows the break instruction to later
be converted without removing instructions inside the parent body. The
extra type coercion instructions go into the body of the br_block_flat,
and backends are responsible for dispatching the instruction correctly
(it should map to the same function calls for related instructions).