compiler: remove impossible cases in jump threading.

* stdlib/optimize.tl (basic-blocks thread-jumps-block): There
can't be any instructions in a basic block after an if or ifq,
so in these cases, jrest is always nil. Let's ignore that nil
efficiently with @nil, and get rid of the cut-block branches
of the code. There is a similar case in peephole-block, but
the target of the jump is an (end ...) which doesn't
necessarily end a basic block. I temporarily put in an (assert
(null jrest)), and, surprisingly, it never went off during a
rebuild of the library or running of the test case. Still,
only a jend ends a basic block; it would not be correct to
simplify it like these two cases in thread-jumps-block.

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