This extractor had a side-effect of invoking `put_in_regs`, which is not
supposed to be invoked until the pattern-matching commits to evaluating
a rule right-hand side (i.e., cannot backtrack). In this case the
side-effect was mostly benign (in theory it could have caused additional
values to be computed needlessly), but in general we should be careful
to keep side-effects out of the left-hand side to enable further
optimizations and work on islec.
The implicit conversion from `Value` to `Reg` turns out to be enough to
make the rules in question work, so we can simply remove the use of the
extractor in this case.
92ce79366c