[llvm-reduce] Create returns with undef values for non-void functions.
always creates ret void instructions if no successor is in the chunk.
This results in invalid IR for functions with non-void return types,
which makes those reductions unfeasible. Instead, create ret ty undef
for functions with non-void return types.
Reviewed By: lebedev.ri
Differential Revision: https://reviews.llvm.org/D86849