Reapply "IR: Add fp operations to atomicrmw"

This reapplies commits r351778 and r351782 with
RISCV test fixes.

llvm-svn: 351850
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 8b67446..f48a970 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -1407,6 +1407,10 @@
     return "umax";
   case AtomicRMWInst::UMin:
     return "umin";
+  case AtomicRMWInst::FAdd:
+    return "fadd";
+  case AtomicRMWInst::FSub:
+    return "fsub";
   case AtomicRMWInst::BAD_BINOP:
     return "<invalid operation>";
   }