Adapt regalloc2-tool to work with creduce

The fuzzing strategy we use for regalloc2 doesn't minimize well, which can make isolating a failure somewhat tedious. If the textual format emitted by regalloc2-tool looked enough like c to be used with `creduce`, we could write a script to automate minimization by using creduce to shrink the input.

Start by locating the textual format emitted by regalloc2-tool and reviewing how creduce consumes inputs. Determine the changes needed for the output to be sufficiently C-like, then verify that a script can use creduce to minimize a failing regalloc2 input.

Rédigé par le modèle d'indexation à partir du texte de l'issue.