[DirectX] Pow optomized to Powi in frotend causes the backend to have an intrinsic it can't lower

[farzonl]

The DirectX backend knows how to lower Intrinsic::pow. It does not know how to lower Intrinsic::powi

This optimization replaces Intrinsic::pow with Intrinsic::powi.

llvm-project/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

Lines 2397 to 2401 in cd56666

	// powf(x, itofp(y)) -> powi(x, y)
	if (AllowApprox && (isa<SIToFPInst>(Expo) || isa<UIToFPInst>(Expo))) {
	if (Value *ExpoI = getIntToFPVal(Expo, B, TLI->getIntSize()))
	return copyFlags(*Pow, createPowWithIntegerExponent(Base, ExpoI, M, B));
	}

We need a change in the backend to handle Intrinsic::powi so we can legalize it to the right thing.