InstCombine: Don't create an unused instruction

We would create an instruction but not inserting it.
Not inserting the unused instruction would lead us to verification
failure.

This fixes PR21653.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222659 91177308-0d34-0410-b5e6-96231b3b80d8