[clang] [llvm] [HLSL] implement elementwise firstbithigh hlsl builtin (PR #111082)

[Farzon Lotfi via cfe-commits]

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@@ -2547,6 +2588,10 @@ bool SPIRVInstructionSelector::selectIntrinsic(Register ResVReg,
     return selectExtInst(ResVReg, ResType, I, CL::rsqrt, GL::InverseSqrt);
   case Intrinsic::spv_sign:
     return selectSign(ResVReg, ResType, I);
+  case Intrinsic::spv_firstbituhigh: // There is no CL equivalent of FindUMsb
+    return selectFirstBitHigh(ResVReg, ResType, I, false);
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farzonl wrote:

```suggestion
    return selectFirstBitHigh(ResVReg, ResType, I, /*IsSigned=*/false);
```

https://github.com/llvm/llvm-project/pull/111082