Get rid of an optimization in SCCP which appears to have many issues. ...

Get rid of an optimization in SCCP which appears to have many issues.  Specifically, it doesn't handle many cases involving undef correctly, and it is missing other checks which
lead to it trying to re-mark a value marked as a constant with a different value.  It also appears to trigger very rarely.

Fixes PR11357.

llvm-svn: 144352