[DAGCombiner] rename function attribute for disabling ftrunc transform

This is the matching name change for the Clang patch at:
D46236
rL331209

Differential Revision: https://reviews.llvm.org/D46237 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331210 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed