[DAGCombiner] refactor FP->int->FP folds; NFC

As discussed in the post-review comments for rL330437,
we need to guard this fold to allow existing code to
keep working with the undefined behavior that they've
come to rely on.

That would mean duplicating more code than we already 
have, so let's fix that first. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330947 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed