[CVP] processOverflowIntrinsic(): don't crash if constant-holding happened

As reported by Mikael Holmén in post-commit review in
https://reviews.llvm.org/D60791#1469765

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358559 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed