Handle operand replacement in JMP_NULL

In this case it's not sufficient to replace the JMP_NULL operand,
as it keeps the temporary alive and there may be more uses later.
Fix this by

Handle operand replacement in JMP_NULL

In this case it's not sufficient to replace the JMP_NULL operand,
as it keeps the temporary alive and there may be more uses later.
Fix this by generalizing existing handling for other similar opcodes
like CASE/SWITCH and LIST_R.

Fixes oss-fuzz 5820123475214336.

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