[SimplifyCFG] Use a more elegant solution than r261731

The cleanupret instruction has an invariant that it's 'from' operand be
a cleanuppad.  This invariant was violated when we removed a dead block
which removed a cleanuppad leaving behind a cleanupret with an undef
'from' operand.

This was solved in r261731 by staving off the removal of the dead block
to a later pass.

However, it occured to me that we do not need to do this.
Instead, we can simply avoid processing the cleanupret if it has an
undef 'from' operand because we know that it will be removed soon.

llvm-svn: 261754