[DAGCombiner] limit ftrunc optimizations with function attribute

As noted, the attribute name is subject to change once we have
the clang side implemented, but it's clear that we need some
kind of attribute-based predication here based on the discussion
for:
rL330437



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330951 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed