Fix TRUNCATE splitting helper logic.

This is a followon to r233681 - I'd misunderstood the semantics of FTRUNC,
and had confused it with (FP_ROUND ..., 0).

Thanks for Ahmed Bougacha for his post-commit review!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235191 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed