This is cherry-picked from #52245. This is an independent bugfix, and
looks like #52245 might need another round of discussion.

There were two separate off-by-1's in the codegen code that is trying to
detect assignments to slots inside try/catch regions.

First, it was asking to include the value of the catch label, which is
actually the first statement *not* in the try region. Second, there was
a confusion of 0 and 1 based indexing in the iteration bounds. The end
result of this was that the code was also looking at the first two
statements of the catch region.

This wasn't a problem before #52245 (other than a potentially
over-conservative marking of some slots as volatile), because our catch
blocks always had at least two statements (a :leave and a terminator),
but with the `:leave` change, it is possible to have catch blocks with
only one statement. If these happened to be at the end of the function,
things would blow up.

As a side node, this code isn't particularly sound, because it assumes
that try/catch regions are lexical, which they are not. The assumption
happens to work out ok for the code we generate in the frontend and
optimized IR doesn't have slots, so we don't use this code, but it is
not in general sound.