Revert r114097, adding back in the assertion against replacing an Instruction...

Revert r114097, adding back in the assertion against replacing an Instruction by itself.  Now that CorrelatedValuePropagation is
more careful not to call SimplifyInstructionsInBlock() on an unreachable block, the issue has been fixed at a higher level.  Add
a big warning to SimplifyInstructionsInBlock() to hopefully prevent this in the future.

llvm-svn: 114117