[llvm-reduce] Create returns with undef values for non-void functions.

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