llvm6: Tweak fast math intrinsics

Looks like they did some refactoring of flags in the backend and this should
catch us up! The "unsafe algebra" boolean has been split into a number of
boolean flags for various operations, and this updates to use the `setFast`
function which should hopefully have the same behavior as before.

This was updated in llvm-mirror/llvm@00e900afd

src/rustllvm/RustWrapper.cpp

@@ -315,7 +315,11 @@ extern "C" void LLVMRustRemoveFunctionAttributes(LLVMValueRef Fn,
 // enable fpmath flag UnsafeAlgebra
 extern "C" void LLVMRustSetHasUnsafeAlgebra(LLVMValueRef V) {
   if (auto I = dyn_cast<Instruction>(unwrap<Value>(V))) {
+#if LLVM_VERSION_GE(6, 0)
+    I->setFast(true);
+#else
     I->setHasUnsafeAlgebra(true);
+#endif
   }
 }