DAGCombiner: Allow the DAGCombiner to combine multiple FDIVs with the same divisor info FMULs by the reciprocal.

E.g., ( a / D; b / D ) -> ( recip = 1.0 / D; a * recip; b * recip)

A hook is added to allow the target to control whether it needs to do such combine.

Reviewed in http://reviews.llvm.org/D6334


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222510 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetLowering.h

@@ -2652,6 +2652,12 @@ public:
     return SDValue();
   }
 
+  /// Indicate whether this target prefers to combine the given number of FDIVs
+  /// with the same divisor.
+  virtual bool combineRepeatedFPDivisors(unsigned NumUsers) const {
+    return false;
+  }
+
   /// Hooks for building estimates in place of slower divisions and square
   /// roots.