Process the -freciprocal-math optimization flag (PR20912)

The driver currently accepts but ignores the -freciprocal-math flag.
This patch passes the flag through and enables 'arcp' fast-math-flag
generation in IR.

Note that this change does not actually enable the optimization for
any target. The reassociation optimization that this flag specifies
was implemented by http://reviews.llvm.org/D6334 :
http://llvm.org/viewvc/llvm-project?view=revision&revision=222510

Because the optimization is done in the backend rather than IR,
the backend must be modified to understand instruction-level
fast-math-flags or a new function-level attribute must be created.

Also note that -freciprocal-math is independent of any target-specific
usage of reciprocal estimate hardware instructions. That requires
its own flag ('-mrecip').

https://llvm.org/bugs/show_bug.cgi?id=20912

llvm-svn: 234493

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -70,6 +70,9 @@ CodeGenFunction::CodeGenFunction(CodeGenModule &cgm, bool suppressNewContext)
   if (CGM.getCodeGenOpts().NoSignedZeros) {
     FMF.setNoSignedZeros();
   }
+  if (CGM.getCodeGenOpts().ReciprocalMath) {
+    FMF.setAllowReciprocal();
+  }
   Builder.SetFastMathFlags(FMF);
 }