Currently replaceBranchTerminator/removeUninterestingBBsFromSwitch
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
