[DAGCombiner] Fold IEEE fmul/fdiv by Pow2 to add/sub of exp

Note: This is moving D154678 which previously implemented this in
InstCombine. Concerns where brought up that this was de-canonicalizing
and really targeting a codegen improvement, so placing in DAGCombiner.

This implements:

```
(fmul C, (uitofp Pow2))
    -> (bitcast_to_FP (add (bitcast_to_INT C), Log2(Pow2) << mantissa))
(fdiv C, (uitofp Pow2))
    -> (bitcast_to_FP (sub (bitcast_to_INT C), Log2(Pow2) << mantissa))
```
The motivation is mostly fdiv where 2^(-p) is a fairly common
expression.

The patch is intentionally conservative about the transform, only
doing so if we:
    1) have IEEE floats
    2) C is normal
    3) add/sub of max(Log2(Pow2)) stays in the min/max exponent
       bounds.

Alive2 can't realistically prove this, but did test float16/float32
cases (within the bounds of the above rules) exhaustively.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D154805

Author: goldsteinn

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -810,6 +810,24 @@ class TargetLoweringBase {
     return !XC;
   }
 
+  // Return true if its desirable to perform the following transform:
+  // (fmul C, (uitofp Pow2))
+  //     -> (bitcast_to_FP (add (bitcast_to_INT C), Log2(Pow2) << mantissa))
+  // (fdiv C, (uitofp Pow2))
+  //     -> (bitcast_to_FP (sub (bitcast_to_INT C), Log2(Pow2) << mantissa))
+  //
+  // This is only queried after we have verified the transform will be bitwise
+  // equals.
+  //
+  // SDNode *N      : The FDiv/FMul node we want to transform.
+  // SDValue FPConst: The Float constant operand in `N`.
+  // SDValue IntPow2: The Integer power of 2 operand in `N`.
+  virtual bool optimizeFMulOrFDivAsShiftAddBitcast(SDNode *N, SDValue FPConst,
+                                                   SDValue IntPow2) const {
+    // Default to avoiding fdiv which is often very expensive.
+    return N->getOpcode() == ISD::FDIV;
+  }
+
   /// These two forms are equivalent:
   ///   sub %y, (xor %x, -1)
   ///   add (add %x, 1), %y