[llvm-reduce] Fix invalid reduction in basic-blocks delta pass

Previously, if the basic-blocks delta pass tried to remove a basic block
that was the last basic block in a function that did not have external
or weak linkage, the resulting IR would become invalid. Since removing
the last basic block in a function is effectively identical to removing
the function body itself, we check explicitly for this case and if we
detect it, we run the same logic as in ReduceFunctionBodies.cpp

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D113486