When InstCombine tries to fold away (fsub x, (fneg y)) into (fadd x, y), it is
also worthwhile for it to look through FP extensions and truncations, whose
application commutes with fneg.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187249 91177308-0d34-0410-b5e6-96231b3b80d8