Instead, use vec::to_mut/from_mut to transform vectors in place as
needed.

There's not a real race here, but it makes helgrind happy and is arguably
less prone to future errrors.

This reverts commit 95099cdd.

This reverts commit c0a5e21b.

This looks like just a random snippet of llasm.

We will generate them ourselves

The prestate for calls was getting set incorrectly to the poststate for the operator
in the call. This worked before since most of the time, operator expressions are
pure. Issue 1895 shows how this breaks when the operator is a closure that has a
move-in capture clause.

(I had a several-day, multi-file patch for this that didn't work... and then it
turned out to be a one-line fix. The joys of programming.)

Closes #1895

Closes #1916