[Reduce] Argument reduction: don't try to drop terminator instructions

[Reduce] Argument reduction: don't try to drop terminator instructions

Newly-added test previously crashed.

While it is up for debate whether or not instruction reduction
should be indiscriminate in instruction dropping (there you can
just ensure that the test case is still -verify'ies), here
if we drop terminator, CloneFunctionInto() will immediately crash.

So let's not do that :)