[llvm-reduce] Skip chunks that lead to broken modules.

Some reduction passes may create invalid IR. I am not aware of any use
case where we would like to proceed reducing invalid IR. Various utils
used here, including CloneModule, assume the module to clone is valid
and crash otherwise.

Ideally, no reduction pass would create invalid IR, but some currently
do. ReduceInstructions can be fixed relatively easily (D86210), but
others are harder. For example, ReduceBasicBlocks may remove result in
invalid PHI nodes.

For now, skip the chunks. If we get to the point where all reduction
passes result in valid IR, we may want to turn this into an assertion.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D86212