Assert/hang in SROA_HLSL pass related to empty base struct regression

#3016 seems to have reappeared. Unknown if it is exact same issue or just similar, but symptoms and general area of code are the same, at least.

Repro is unchanged:

Compile the following with -E main -T vs_6_0:

struct EmptyStruct
{
};

struct BaseStruct
{
	EmptyStruct emptyStructMember;

	//float makeMeNotEmpty;	// Uncomment to prevent assert
};

struct ChildStruct : BaseStruct
{
	float4 m_childMember;

	float4 func() { return m_childMember; }
};

float4 main() : SV_Position
{
	ChildStruct var_1;
	EmptyStruct var_2;
	var_1.emptyStructMember = var_2;	// Comment out to prevent assert
	var_1.m_childMember = 0;
	return var_1.func();
}

Reproduce the assertion with the supplied HLSL input using -E main -T vs_6_0, then inspect the SROA_HLSL pass and compare the behavior with issue #3016. The fix should allow this empty-base-struct case to compile without an assertion while preserving the existing shader output behavior.