[SCCP] Rename undefined -> unknown.

In the solver, isUndefined() does really mean "we don't know the
value yet" rather than "this is an UndefinedValue". Discussed with
Eli Friedman.

Differential Revision:  http://reviews.llvm.org/D22192

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275004 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed