[Hexagon] Return the correct chain edge for i1 function calls

In HexagonISelLowering, there is code to handle the case when
a function returns an i1 type. In this case, we need to generate
extra nodes to copy the result from R0 to a predicate register.

The code was returning the wrong value for the chain edge which
caused an assert "Wrong topological sorting" when converting the
instructions to MIs.

This patch fixes the problem by returning the chain for the final
copy.

Patch by Brendon Cahoon.

llvm-svn: 316367