When propagating equalities, eg replacing A with B in every basic block

dominated by Root, check that B is available throughout the scope.  This
is obviously true (famous last words?) given the current logic, but the
check may be helpful if more complicated reasoning is added one day.

llvm-svn: 153323

lib/Transforms/Scalar/GVN.cpp

@@ -2001,6 +2001,9 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS, BasicBlock *Root) {
       LVN = RVN;
     }
   }
+  assert((!isa<Instruction>(RHS) ||
+          DT->properlyDominates(cast<Instruction>(RHS)->getParent(), Root)) &&
+         "Instruction doesn't dominate scope!");
 
   // If value numbering later deduces that an instruction in the scope is equal
   // to 'LHS' then ensure it will be turned into 'RHS'.