[llvm] [DXIL] Implement pow lowering (PR #86733)

[Farzon Lotfi via llvm-commits]

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@@ -197,6 +198,26 @@ static bool expandLog10Intrinsic(CallInst *Orig) {
   return expandLogIntrinsic(Orig, numbers::ln2f / numbers::ln10f);
 }
 
+static bool expandPowIntrinsic(CallInst *Orig) {
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farzonl wrote:

Its a good question the reason why is  `expandIntrinsic` I need to be able to return true for the intrinsics that are expanded so if we made these void returns then
```c++
switch (F.getIntrinsicID()) {
  case Intrinsic::abs:
    expandAbs(Orig);
   return true;
  ...
  case Intrinsic::pow:
    expandPowIntrinsic(Orig);
    return true;
```
Having all those return trues in the switch statement was just ugly so i moved them into the function bodies. 

https://github.com/llvm/llvm-project/pull/86733