Try to fix an SDAG dependence issue with sret

r208453 added support for having sret on the second parameter.  In that
change, the code for copying sret into a virtual register was hoisted
into the loop that lowers formal parameters.  This caused a "Wrong
topological sorting" assertion failure during scheduling when a
parameter is passed in memory.  This change undoes that by creating a
second loop that deals with sret.

I'm worried that this fix is incomplete.  I don't fully understand the
dependence issues.  However, with this change we produce the same DAGs
we used to produce, so if they are broken, they are just as broken as
they have always been.