[region-isolation] Fix handling of coroutine apply results.
In this part of the code, we are attempting to merge all of the operands into
the same region and then assigning all non-Sendable results of the function to
that same region. The problem that was occuring here was a thinko due to the
control flow of the code here not separating nicely the case of whether or not
we had operands or not. Previously this did not matter, since we just used the
first result in such a case... but since we changed to assign to the first
operand element in some cases, it matters now. To fix this, I split the confused
logic into two different easy to follow control paths... one if we have operands
and one where we do not have an operand. In the case where we have a first
operand, we merge our elements into its region. If we do not have any operands,
then we just perform one large region assign fresh.
This was not exposed by code that used non-coroutines since in SIL only
coroutines today have multiple results.
rdar://132767643
(cherry picked from commit 541863dbc67d83674979f2dfa4ab836cda93330c)
