lib/CWriter/Writer.cpp: Copy AsmWriter's ConstantFP checking code here

lib/CWriter/Writer.cpp: Copy AsmWriter's ConstantFP checking code here
into a new function FPCSafeToPrint(), and use it in printConstant()
and printFunction() to decide whether we should output ConstantFPs as
floating-point constants or as references to stack-allocated variables.

lib/VMCore/AsmWriter.cpp: Fix an apparent typo in the code mentioned above.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6762 91177308-0d34-0410-b5e6-96231b3b80d8