Incorrect behaviour when using Buffer with struct template parameter.

[neversleeping]

Description
When using Buffer with struct template parameter (like Buffer<SomeStructName>), compilation should fail with error telling that Buffer supports only scalar, vector and matrix types as template parameter. But instead:

1. When compiling to DXIL compilation just succeeds.
2. When compiling to SPIRV with enabled optimizations compilation will either succeed or throw exception "Access violation reading location".
3. When compiling to SPIRV with disabled optimizations compilation will result in flowing error:

fatal error: generated SPIR-V is invalid: Reached non-composite type while indexes still remain to be traversed.
  %21 = OpCompositeExtract %uint %20 0

Maybe, somewhat related to #7080, but not sure.

Steps to Reproduce
Compile with -E CSMain -T cs_6_6 -spirv -Od

struct Struct {
    uint member;
};

Buffer<Struct> buffer;

[numthreads(32, 1, 1)]
void CSMain()
{	
	Struct var = buffer[0];
}

https://godbolt.org/z/MGchz4K3E

This code is specifically for producing error with bug request on SPIRV compilation. It's harder to make reduced example which will lead to exception when compiling with optimizations, but this exception is caused by the same thing, because when I'm replacing Buffer with StructuredBuffer, it goes away.

Actual Behavior
Compilation in both DXIL and SPIRV should result in error that Buffer can't be used with struct template parameter.

Environment

• DXC version: 1.8 - 1.8.0.4666 (release-1.8.2407, 8df0a9d)
• Host Operating System: Windows 11 23H2