[IndVars] Fix corner case with unreachable Phi inputs. PR40454

Logic in `getInsertPointForUses` doesn't account for a corner case when `Def`
only comes to a Phi user from unreachable blocks. In this case, the incoming
value may be arbitrary (and not even available in the input block) and break
the loop-related invariants that are asserted below.

In fact, if we encounter this situation, no IR modification is needed. This
Phi will be simplified away with nearest cleanup.

Differential Revision: https://reviews.llvm.org/D58045
Reviewed By: spatel


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353816 91177308-0d34-0410-b5e6-96231b3b80d8