add the -mrecip driver flag and process its options

This is the front-end counterpart to D8982 (LLVM r238051).

The -mrecip option interface is based on maintaining compatibility with gcc:
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/i386-and-x86-64-Options.html#index-mrecip_003dopt-1627
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/RS_002f6000-and-PowerPC-Options.html#index-mrecip-2289

...while adding more functionality (allowing users to specify the number of refinement steps for each
estimate type).

Differential Revision: http://reviews.llvm.org/D8989

llvm-svn: 238055

clang/lib/CodeGen/BackendUtil.cpp

@@ -480,6 +480,9 @@ TargetMachine *EmitAssemblyHelper::CreateTargetMachine(bool MustCreateTM) {
 
   llvm::TargetOptions Options;
 
+  if (!TargetOpts.Reciprocals.empty())
+    Options.Reciprocals = TargetRecip(TargetOpts.Reciprocals);
+
   Options.ThreadModel =
     llvm::StringSwitch<llvm::ThreadModel::Model>(CodeGenOpts.ThreadModel)
       .Case("posix", llvm::ThreadModel::POSIX)