BasicBlock::getTerminator() is frequently called on valid IR, yet the
function has to check that the last instruction is in fact a terminator,
even in release builds. This check can only be optimized away when the
instruction is dereferenced.
Therefore, introduce the functions hasTerminator() and
getTerminatorOrNull() as replacement and require (assert) that
getTerminator() always returns a valid terminator. As a side effect,
this forces explicit expression of intent at call sites when unfinished
basic blocks should be supported.
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