llvm-reduce: Preserve frame index values when cloning function

Previously the specific values used for fixed frame indexes was in
reverse order in the cloned function from the original, and a map was
used to adjust all frame indexes to the potentially new values. Insert
the fixed objects in reverse to avoid this. This simplifies other
code, since now we don't need to track down all frame indexes. This
will allow targets that store frame indexes in MachineFunctionInfo to
simply copy the values.

Note this isn't directly observable in the test since the resulting
MIR print/parse can shuffle the IDs around (in particular the final
serialization implicitly strips out dead objects).