[CodeGen] match new fast-math-flag method: isFast()

This corresponds to LLVM commiti r317488:

If that commit is reverted, this commit will also need to be reverted.

llvm-svn: 317489
diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 234d1a2..3b60b99 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -87,7 +87,7 @@
 
   llvm::FastMathFlags FMF;
   if (CGM.getLangOpts().FastMath)
-    FMF.setUnsafeAlgebra();
+    FMF.setFast();
   if (CGM.getLangOpts().FiniteMathOnly) {
     FMF.setNoNaNs();
     FMF.setNoInfs();