Add an assertion to catch indirectbr in SplitBlockPredecessors. This

makes several optimization passes abort in cases where they're currently
silently miscompiling code.

Remove the indirectbr assertion from SplitEdge. Indirectbr is only
a problem for critical edges, and SplitEdge defers to SplitCriticalEdge
to handle those, and SplitCriticalEdge has its own assertion for
indirectbr.

llvm-svn: 86147