[DeadArgElim] Split the invoke successor edge

Invoking a function which returns an aggregate can sometimes be
transformed to return a scalar value.  However, this means that we need
to create an insertvalue instruction(s) to recreate the correct
aggregate type.  We achieved this by inserting an insertvalue
instruction at the invoke's normal successor.  However, this is not
feasible if the normal successor uses the invoke's return value inside a
PHI node.

Instead, split the edge between the invoke and the unwind successor and
create the insertvalue instruction in the new basic block.  The new
basic block's successor will be the old invoke successor which leaves
us with IR which is well behaved.

This fixes PR24906.

llvm-svn: 248387