Add support for fast-math flags to the FCmp instruction.

Add support for fast-math flags to the FCmp instruction.

FCmp behaves a lot like a floating-point binary operator in many ways,
and can benefit from fast-math information. Flags such as nsz and nnan
can affect if this fcmp (in combination with a select) can be treated
as a fminnum/fmaxnum operation.

This adds backwards-compatible bitcode support, IR parsing and writing,
LangRef changes and IRBuilder changes. I'll need to audit InstSimplify
and InstCombine in a followup to find places where flags should be
copied.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241901 91177308-0d34-0410-b5e6-96231b3b80d8