fix PR6940: sitofp(undef) folds to 0.0, not undef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102358 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/SCCP.cpp

@@ -1452,6 +1452,8 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
         // After a zero extend, we know the top part is zero.  SExt doesn't have
         // to be handled here, because we don't know whether the top part is 1's
         // or 0's.
+      case Instruction::SIToFP:  // some FP values are not possible, just use 0.
+      case Instruction::UIToFP:  // some FP values are not possible, just use 0.
         markForcedConstant(I, Constant::getNullValue(ITy));
         return true;
       case Instruction::Mul: