Don't do pre-splitting if doing so would create a value join that did not

Don't do pre-splitting if doing so would create a value join that did not
exist before.  Updating the live intervals in that care is tricky in the general
case.

Evan, if you see a tighter guard condition for this, let me know.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58560 91177308-0d34-0410-b5e6-96231b3b80d8