[NFC][clang] Replace unchecked dyn_cast with cast (#98948)

Summary:
BI__builtin_hlsl_elementwise_rcp is only invoked with a FixedVectorType
so use cast to make this clear and satisfy the static verifier.

Test Plan: 

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Tasks: 

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Differential Revision: https://phabricator.intern.facebook.com/D60251454

clang/lib/CodeGen/CGBuiltin.cpp

@@ -18389,13 +18389,12 @@ Value *CodeGenFunction::EmitHLSLBuiltinExpr(unsigned BuiltinID,
       llvm_unreachable("rcp operand must have a float representation");
     llvm::Type *Ty = Op0->getType();
     llvm::Type *EltTy = Ty->getScalarType();
-    Constant *One =
-        Ty->isVectorTy()
-            ? ConstantVector::getSplat(
-                  ElementCount::getFixed(
-                      dyn_cast<FixedVectorType>(Ty)->getNumElements()),
-                  ConstantFP::get(EltTy, 1.0))
-            : ConstantFP::get(EltTy, 1.0);
+    Constant *One = Ty->isVectorTy()
+                        ? ConstantVector::getSplat(
+                              ElementCount::getFixed(
+                                  cast<FixedVectorType>(Ty)->getNumElements()),
+                              ConstantFP::get(EltTy, 1.0))
+                        : ConstantFP::get(EltTy, 1.0);
     return Builder.CreateFDiv(One, Op0, "hlsl.rcp");
   }
   case Builtin::BI__builtin_hlsl_elementwise_rsqrt: {